Does the campaign difficulty setting reduce income and public order?

[Ludens]

The difference between the building card and the actual income from mines is a known issue inherited from vanilla RTW. I do not think the difficulty setting has anything to do with it.
H or VH in campaign difficulty raise the aggresiveness of rebels. They will actually attack your towns.

Correct. VH will also give a 10.000 mnai bonus to A.I. factions every turn.