My position is not that the Russians can win. They cannot. My position is that if the Russians keep fighting, the Germans collapse before the Russians do, since Germany has to keep millions of men in Russia to occupy the land and fight Worker-Partisans. This means the Western Front of Germany does not get millions of men of reinforcements that happened in our timeline in 1918 due to the Russians surrendering. What this means is with American reinforcements, the Allies shatter the German lines even faster, Germany would be facing an invasion into western Germany by summer of 1918.
This logic doesn't hold up. Brest-Litovsk treaty was signed in March 1918. German spring offensive started in March 1918. The german 17th, 18th and 19th armies on the Western front were estabilished in late 1917 and early 1918 to organise the 50 freed divisions coming from the east. The divisions weren't freed by the treaty, but by the total disintegration of any organised resistance to the german arms in the east.
Historically, Bolsheviks were trying to do exactly what you propose. They were hoping to drag the war long enough for Germany to collapse. Not to help the rest of the Entente, but to help foster the proper proletarian revolution. According to at-the-time conventional Marxist thinking, true communist revolution had to happen in a country with an industrial economy. Russian Bolsheviks waited for german working class to rise up and overthrow the Kaiser. The two brotherly revolutionary regimes would then make white peace. Or status quo ante bellum peace. This "No war, no peace" was IIRC a brainchild of Trotsky, and it failed spectacularly. German proletariat failed to rise up in time and Bolsheviks were forced to surrender a massive piece of land to preserve their control (however tenuous) of Russian core territory.
To illustrate why any defense against Germany was unrealistic in late 1917 and early 1918, let's have a look at Petrohrad. A city of 700 000 people then. You mentioned a need to siege it. Let's try to have a look at the balance of forces.
In October 1919, after the Bolsheviks had in theory had already two years to consolidate their regime, the city was threatened by general Yudenich with about 20 000 men armed by the Brits. They had some tanks even. Petrohrad Reds were stricken by a panic and were ready to abandon the city, until Trotski personally took charge, armed every available Red supporter and railed in reinforcements from Moscow. Those desperate measues enabled the Red army in Petrohrad to grow and outnumber Yudenich's force by three to one (!). The Red army in the highest hour of need using every trick in the book and defending the most industrialized city in Russia, the very heart of its revolution where it had the most supporters, could muster some 60 000 men (well, fighters). Equivalent of four divisions. Well, that's a bit misleading, because a division implies some organised force with organic artillery and other support arms, capable of combined arms battle and of manoeuvering while keeping cohesion. These 60 000 militias were very unlikely equivalent to actual four divisons of proper troops. Some sources say Yudenich suffered losses already, so 60 000 is an optimistic estimate, it was likely less.
Central Powers had, even after the massive reshuffle westwards, 53 divisions available to launch an offensive in February 1918 (this was the advance that made Bolsheviks sign Brest-Litovsk). And the 53 divions were exhausted, and at the tail end of long supply lines - but they were properly organised and trained troops. Nothing the Reds could muster could stand against that kind of force, if it was to advance further. To illustrate the disparity further, the northern prong of the last german offensive was composed of 16 divisions. As we've seen, Reds could conceivably conjure 4 "divisions" of pathetic militia (I don't want to disparage the bravery of the men and women thus mobilized, but they were no match for a proper army). Any siege of Petrohrad would be a foregone conclusion.
EDIT:
Also your belief that the Communist Russian army did not exist or have offensive power is false, as the Bolsheviks retook Kiev in February 8th 1918. In spite of the fighting with White Russian forces.
The counter-revolutionary forces were all in all even more pathetic than the Reds in some places, that's true. But that doesn't change the fact thet Read Army was in no condition to fight against serious opposition (see Baltics, Poland, Finland...)
