Here is some responses to your questions Llyad :
1) Yes! Your armies accumulate xp throughout the different battle, and at each step of the campagn ou have the choice to select which one you use in the battle and which one you keep in reserve. Moreover during battle you can call for reinforcement some units in reserve
it is a nice idea.
2) I can't answer you because I only played in normal mode. But I fear that, like the majority of the games, the hard mode is only a boosted ia unit mode. Hopefuly the ia is very correct in normal mode, and this cumulate to the difficulty of the campaign (very very well calibrated I think) give us a very good challenge!
3) A very big Yes! I don't remember if in the demo it is obvious but trust me, you will feel the necessity to well organize your unit with the terrain caracteristic very soon in the campaign
I thank again the dev for this very very good game
(too bad that there is not yet a true skirmish mode, but I trust the dev to incorporate in via a patch very soon, pleaaaase)