Conquer Mode would be a great and needed addition. Would help reduce fat finger errors & actual usability/playability of the game.
'Undo' is just not gonna happen, as you said, it could be abused. At best it would be limited to some # of uses per day, and maybe even would be an Advancement ... or would just be like 'Reset AP' where it's always available but costs coins to do.
But if you think about it, there's no easy way to do it. I'm pretty sure that the Idle save points save only the current state of everything, not how you got to that point. So once you (for example) swap in/out some artifacts, use 1 SP SAC + 2x regular SAC powers (total 1hr) on a single army camp, buy some mercs & capture a territory using TS, all the game knows is that you have 20x generation on a single army camp for 1 hr but not whether you used powers or an a Rare artifact, that you have some # of armies left & 1 more territory is captured but not whether you captured it normally or with TS/QS, whether HB or FH were in play, etc.
So it's quite a big ask to have an 'Undo' of any sort beyond going back to the last save point itself, which is potentially too big an Undo, b/c it could be hundreds of actions, not just 1 or 2.
But let's say Undo is implemented, would you be willing to pay 25 coins per undo? 50 coins? I'm guessing at what the price would get set at. If AP reset costs 300, and the powers cost TW 100, SAC 50, SM 20, FC 50, MR 50, FB 40, IM 150, SL 200, ML 700 ... an Undo could cost anywhere from 10-100 coins, I'm guessing. Hard to gauge, b/c sometimes the costs like ML @ 700 & SL @ 200 & AP reset @ 300 are imho ridiculous, compared to the relatively more reasonable SAC, SM, FC (although arguably you need 5 to be truly meaningful, so effectively it becomes 250), FB (but again you probably need 5+ to be truly useful, so price is really 200+). IM is borderline but it's one of the truly most powerful if used wisely.