They dont track population growth, because the game focuses on such an short period, even tho you can keep playing of course, which doesnt really make it worth it to put it in. The manpower growth simulates people reaching conscription age however.
Canada and australia are independent, even tho they start in the allies with UK. India however is an puppet state, called British Raj, so conscription laws wont affect them and you wont get much manpower that way anyway, because non-core states only give 10% of the manpower. Same goes for africa and in the case of WWW part 4 for the rest of france, now under british controll.
No it don't track population changes. The extra manpower gain from getting harsh conscription laws must be counted against the political point cost and the production loss.
Well then that means there is very little to no benefit to staying at anything other than total conscription, since your available manpower only grows at optimum rate if you're actively conscripting everybody, apparently. Yay realism!