1.) The Bitcoin mining difficulty is not something static. If mining become easier or less expensive through AI, the bitcoin algorithm and it’s economics will quickly increase the difficulty. See https://en.bitcoin.it/wiki/Difficulty and https://blockchain.info/nl/charts/difficulty.
2 i.) Yes these type of security issues, are more likely to affect long (months / years) running smart contracts.
2 ii.) Smart contracts actually hold and control assets (like money). Putting in some mechanism to move those assets to an updated contract has the same effect as putting in some mechanism to modify the contract. This means the contract is no longer immutable. Immutability is a key feature of smart contracts.