If you're looking for a "minimum exploit-preventing cheat" I'd personally go for retroactive morale. When the AI receives a war declaration, let it move its maintenance up immediately and bump up (or even max out) its morale immediately instead of waiting for the month tick.
I'm sympathetic to the idea that it's hard to program an AI to spot ambushes, so putting in some ambush-proofing is fair. Free forts forever seems a little extreme.
If we do get that then AI allies should be obliged to let us know when they are about to go to war and call us in. otherwise we could be exposed to the full morale on a low morale stack exploit from the AI esspecially if an OPM with no chance to escape it. It might actually make more historic sense since if you are being attacked wouldnt you want to fight back more as you see in unjust or just to protect yourself and your country but it would give the defender a huge advantage over the attackers until several months into the war.
- 3