I think there is a logical explanation for the super peasants...

i. When a revolt of something occurs it creates rebels troops of X value...

ii. Now if you have destroyed the other building the only units it can build are peasants,

iii. There is some limit to the number of troops that can be created.

Put those together and the only way you can make up the value of your limited number of peasant troops is by purchasing them with valour and other upgrades (just like in a skirmish game)...

As for buildings in a single turn I will have to check...