You get 0,1 Gildar from every citizen per turn. More pop = more tax. But you can not raise or lower taxes. But as you wrote, you can raise your tax income by improvements.
Mana: There is a talent that gives you +2 mana per turn (attunement). And you can increase the mana income by shards with researching bigger shrines (magic - techtree).
I'm still not sure I understand, is it for each citizen created in the city, or each citizen used in the city?
Correct me if I'm wrong, but since the population is a global resource, it is possible to have a very populated city without many buildings, and a small city with more buildings than citizens.
In other words, if I have a city dedicated for gold, should I invest in its size or just build gold-giving buildings and gain the population from a different city?
(and yes, I know I should also improve the city to get the gold % bonus, this question is only to clarify my confusion)