It's a bit of simplification. There are 3 other major centers which you omitted:
- Novgorod. A city and state that was distinct because of foreign connections, own thriving trade and had a local culture and separate identity pretty much since times of Kiev's ascendancy. It fell to Muscovy and was pretty much destroyed in subjugation, old lucrative trade being cut off and other things. The identity of Russian North was more or less settled when Peter I created new capital, Saint Petersburg, which itself was a massive cultural and demographic influence, focused on itself a lot of trade routes and thus absorbed the remnants of the Novgorodian identity, as well as other natives (incluing many non-Slavic).
- Chernigov (and at some point Ryazan) was another major area that was prominent in late Kievan Rus times. Being powerful and distinct, it was also one of the worst suffered regions to Mongols. It was later assimilated by XVIII century by two processes - one was Muscovy colonization of the southern areas (founding towns like Kursk and others only in XVI century as forposts) and... Chernigov was made Ukrainian during the Cossack civil wars, when Right-bank Ukraine's population was transferred to the Left bank, among them to relatively empty Chernigov area as well where they assimilated locals by numbers, creating such a divide there. It's also a reason of vast Ukrainian presence to the Eastern Steppes, it was a result of forced migrations in XVII-XVIII centuries to the land with low population.
- Halych-Volhyn. It was actually a powerhouse in the immediate situation when Kiev was destroyed and until middle of XIV it was a serious pretender to the power in region. However, some losses and then unfortunate death of rulers with no succession beheaded the duchy/kingdom and it was partitioned by two strong neighbors in a long war over the succession over land which was untouched by Mongols. Lviv had even a strategic importance - in XIV-XV centuries through Dniester to Lviv and then through Krakow one of the major branches of the Silk Road went, making a reason why that land was that developed at the time, being better than Muscovy for a while. Although I want to note that Western Ukrainian identity didn't disappear at that point at all, unlike the other two. It remained.
Another thing you miss, and one which was most crucial to the separation of Belarus and Ukrainian identities, were the Cossack Wars of XVII century, which made a profound impact on culture, politics and society in Ukraine. Up to that point Belarusians and Ukrainians were, more or less, considered Ruthenians as a whole, Orthodox people of the Commonwealth. Even if nominally divided by border, it didn't matter that much. The Cossack Rebellions and wars that engulfed most of Ukraine changed it completely, creating a whole new class of people while ousting old polonized elites and imprinting themselves in culture, folklore, architecture, history. Ukranians strived for the ideals of Cossacks, and at the time it excluded most of Western Ukraine and Belarus. But the impact they had on culture and nation was important, it was something that persisted even after the ideal Cossacks were gone and landlords were again ruling over peasants. Later, under different circumstances and different interpretation, similar ideals were picked up by Western Ukraine.
In Belarus nothing like this happened. It's evident by many things in their culture, self-perception and how they nowadays idealize Polish/Lithuanian days, perceive their state as a simple continuation of that period, present a cultural alignment to Poland, Lithuania and Russia... and, well, being driven by perception of being "locals" in the first place, not someone too distinct from neighbors. As a group that is living in that land that was kinda always there, didn't assimilate with others but also didn't really mind assimilation. Hence why the national ideas of Belarus are so weak. Yet they have their own identity of sorts which prevented them from becoming full-fledged Lithuanians/Poles/Russians. Even now, with most of them speaking Russian as primary native tongue, their identity with being Belarusian, local, is strong. A lot thanks to the aforementioned split of groups and them being a distinct group in a place that wasn't easily accessible or too desirable.
Not really, it happened much later under Soviet rule. Large swathes of Southern Russia spoke Ukrainian, there were literally German lands, relatively more minorities than there used to be. In fact, there were large swathes of land in Far East where Ukrainian was a dominant language even at a time.
While I do agree about the paragraph part about the weakness of state apparatus and primary education, I do have to ask... Austrian's evil plans about cultivating Ukrainian identity?
The major reason why Austrians let it grow was because they saw a rivalry between two sides, Poles and Ukrainians, as very beneficial to ruling over the Galicia. Obviously, they didn't want Ukrainians there to identify as Russians for understandable reasons and so they preferred Ukrainians. However, until WWI there was a very strong pro-Russian identification movement which mattered and wasn't even repressed (at least much more than pro-Ukrainian one)... which ended up being annihilated, ironically by Imperial Russia during occupation of the Galicia, it suspected them being "filthy Austrian spies", a lot of heavy uncalled for repressions happened... and then Russia started to lose war and had to retreat, with Austrians coming back and shooting Russophiles who were spared. It destroyed Russophile faction and politicians, making much bigger work for cultivation of Ukrainian identity than Austria in a way.
But there was another major issue why Austria played such a major role: Imperial Russia didn't let pro-Ukrainian politicians and activists be, repressing them a lot. For many Austrian-controlled Ukraine became a safe haven - because with all downsides, censorship was nowhere near as bad in Austria, it was relatively a liberal paradise compared to conservative Russian Empire. Two intellectual elites in XIX century, from Western Ukraine and rest of Ukraine, heavily intertwined in a result, united by a quest of liberation against oppressors. Western Ukraine saw rest of Ukraine as a major backup against the Poland while other part of Ukraine used liberalism in Western Ukraine under Austrian rule. For XIX century, when literature Ukrainian (adopted by both literature elites) was created and process of national identification happened, it was a major factor why Western Ukraine and rest of Ukraine united together. There wasn't even much alternative - in Russia Ukrainian was banned, identifying politically as Ukrainian politician was not possible.
The talks about "Austria cultivating" are just an excuse. It's running away from resposibility for "failure" of Russian Empire.
In 1920s, when they were created, there was a different logics behind it.
USSR was ravaged by years of civil war, devastated, with massive elements opposing the Soviet rule, a lot of potentially rebellious provinces and branches of socialists and such that were not too happy.
What happened is that Lenin decided to create national republics - run by more or less happy and allied to Moscow politicians. They were quite sovereign, in 1920s launched massive education campaigns in local languages and had a major cultural boost. It made pretty much all potential separatist regions from being rebellious to being loyal to Moscow, becoming from weakness a power. Furthermore, they were interested in opposing "white" elements and counterrevolution inside of them, being crucial for Soviet order and it managing to withstand in harsh conditions. It allowed to destroy counterrevolution in Russia itself and cement Soviet rule... as well for Stalin later paving a way to create a totalitarian state in a similar fashion, later hijacking the republicks and making them small minidictatorships integrated in whole totalitarian system very well. A lot of poets, writers and such in 1930s were killed or purged in Russia, Ukraine and other places. That period in Ukraine (1920s, peak of cultural revival) is called Executed Renessaince.
The total Russification started to happen only post-war, becoming total in since 1960s as it wasn't a focus before that. Russian schools were forced even in old districts (Ukrainian converted to Russian), universities were Russian-only, the public language defacto was Russian (became the only state language of USSR in 1980s).
This one is also false. There wasn't any new identity born out of thin air or claimed. National identities existed in USSR as well, although it did try to eridicate them. They were far from being gone or becoming marginal even in most optimistic cases.
Rather, the Soviet Union wanted to create a common Soviet identity which was disenfranchised from land and origin, which would absorb all. But it backfired and failed. Even Russians didn't accept it and craved for their own identity (even if ironically now they consider Soviet identity to be part of Russian identity now), hence why person like Yeltsin would appear in the first place. It also must be important that Soviet identity was spread on Azeris, Armenians, Kazakhs, Tatars and many many other very different non-Slavic nations.
No, it doesn't. What made them different from Ukraine and why it diverged as a branch? Defacto (written) Ruthenian at that point was similar/same in Ukraine and Belarus. It's only 19th century when literature language split happens.
Outside of answers that were provided there is one more major answer: economics and economic make-up of regions.
Until Soviet times, Ukrainian economy wasn't really that aimed at all-Russian market. In fact, it was the producer of many resources, exported and had it's own major gateway into the Russian Empire - Odesa. The economic links kept people integrated as economics worked in a region being a major producers and sustainable in economic sense. Belarus... was relatively a rural part with not much connections instead.
Soviet times saw a much closer economic integration of all republics, aimed by state on purpose as well. It was also a period a bigger assimilation and integration of regions. However, the soviet system proved to be unsustainable so it collapsed anyway. Most of ex-USSR states drift apart economically regardless of relations because of the healthy economic interests of most countries.