AOE II Campaign

Hello! I suck at strategy games, multiplayer ones, but I have fun trying to play them as single player. I own the AoE II HD Edition on Steam but don't own any of the expansions, yet I plan to buy them if I find them on sale (my country makes spending on games look quite expensive). I have a few questions

1. Is the AI cheating? Heard it was at some point.
2. Do balance changes for expansions affect the campaigns that came before them? If yes, do I have to own them for changes to take place?
3. How worth it are the expansion campaigns? I love the game when I was 10 years old, but mostly did just custom maps and cheated myself in the campaigns.

Thanks for the help!