1. If AA has introduced a direct flight and you choose to take that, I can't imagine why they wouldn't charge you $75 for the re-issue. If that saves you a $150 extra segment, you'll net the $75 difference.

2. Yep, except it probably isn't a major hassle. You give them your ticket and some time later they give you a new ticket. You're right about rule changes if there are any of importance since the first issue.

3. You can add or subtract segments later. Afaik you can add or subtract continents later, but someone who really knows should pontificate on that one. Whether or not it's a hassle at any time is pretty much at the whim of the rate agent.