The level of undo which has been available in games for the last three months is fine for opponent against opponent. The addition of an undo from supply to the action phase made things a little less stringent on players, but no-one playing on a platform like this should expect anything other than the stringent 'touch-move' one encounters in formal chess. If you screw up, you screw up, the game is long enough no individual action will decide it. And depending on player honesty would be a disaster - just look at Hanabi on this site, which is a sufficient mess I would never play the game online.
I am amazed that you were ever willing to manually undo a players move

I've reported a lot of bugs but I would never have even considered asking for a mistake to be corrected - that is clearly not a good use of your time.
Having a full undo system in solo games would be a brilliant analytic tool but I cannot imagine that it is the highest priority - there are at least two actual game rules (home defence and beach interception) which are not implemented, and some weird bugs with supply (which you have explained the difficulty in fixing).
Perhaps there is something that I am not seeing but without the undo patch Main Event games appear to be stable (apart from the occasional mis-attribution of a coastal hex). I would be inclined to let the game stand for a while. From the outside this looks like you are investing a frustratingly large amount of effort for a minimal return - noone wants you to burn out before you get to your promised next project, and you should probably take a moment to reflect on having successfully delivered the largest most complex game BGA has ever seen, a game substantially more complex than the site was ever intended to support, that is a major achievement.