1. Once the miles are in your FF account, the credit card company would have a very hard time rescinding them (in most cases). In your case, no issues.

2. Often it is worth downgrading the card to a no fee version to keep average account age up (and your credit score up). I'm not sure of the time for re-applying for Amex cards before being eligible for the bonus again, but 2 years is probably a safe bet.

3. Not sure on this one. Some cards are from the date it was opened, others are from the date it was closed. Someone else will have to answer.

4. This is all opinion and depends on how you value miles. Most people have a specific airline or two they prefer, so different offers are more lucrative than others. I generally jump on offers that are 50k+ though.