I was also struck by this exception but i thought it irrelevant to subject because their manpower is represented good more or less through number of million peoples they had at time.I am worried for nations(people) which millions are not represented in game.
2nd-Uk or ge through releasing new liberal or fachist nation (with modified manpower) and giving equipment to form new armies could help their exshausted manpower potential
True peoples are not growing on tree as you said and total manpower is not accumulative-you have a point.But i have one word in defense-high reproduction levels in those times-several men reach service one becomes too old.Raising manpower though events sound historical and more precise but it is too complex i think.
That slider would be logic (as it isnt on present state) if it would give better bonuses to proffesionals and dramatic effects to income of manpower-10% or less income than regular.Now that slider strongly favorise professionals although description of slider say that liberals want cheap drafted armies (cheap in words they are not constantly paying huge proffesional army in times of war and peace) i dont see why are they cheap.The gearing bonus is just not enough to represent benefits of drafted armies.
I would propose slider which would give higher bonuses on military scale(better land doctrine research,battle event bonuses...) for profs but dramaticly less manpower and higher upkeep-while drafting would lower org,mor bonuses in favor for larger number of conscripts and less upkeep.
Yugoslavia would i am sure(at least serbian part),poland would.Paraguay would like in gran chaco war against all neighbours

.All nations(thats maybe problem for africa with no or litle national concience in that time) with strong feeling of insecurity of their fate would fight to the last.Nacist ideology of extinction of other people would drive those people to sacrifice everything they can against invaders if they could stay in the field and keep their freedom.
Game allow this thing to change through research building ic and other things.You are denying playability to some countries (which can be released) based on strict historical timeline-thats not this game is for.If luxemburg can have divisions in field than africans will have divisions if i release them with italy give them equipment or improve their industry.Both of things are ahistorical but this game is for fun not just strictness to history timeline.