1. no
2.1 will fail instantly but will retry when the transaction expires. If the proposal get approved again by disapprove then approve, it will try to execute again.
2.2 nothing will happen when only balance changed.
3. need an authorized account to pay transaction fee. It can be any account. for example, can be temp-account, in this case don't need additional signature, but need to transfer some asset to temp-account, for example from another account or from blind balances which will need one signature or more.
4.yes.