An attacker could send coin to someone, receive something of value in return, wait n confirmations, and double-spend the transaction via an alternate blockchain. I would consider this to be stealing.
Yes. But you have to have control of the inputs that were used in the transaction to double spend. Quote appears to say that you can just randomly siphon funds from other addresses.
Depending on how severe the 51% attack is, an attacker could direct their own equipment towards a pool they control and get miners to mine on their pool via higher actual mining earnings. The pool would have higher mining earnings because it wouldn't have its blocks orphaned.
At the end of the day, miners probably won't support the attacker. After all, the miners would be losing money after any 51% attack. The rational thing miners would do is to try to mitigate the 51% attack or to switch off their equipment. Helping the attacker would just be burning through your cash.
Honestly, if someone were to 51% attack Bitcoin, I doubt they will be honest enough to let me get the block rewards.