By telling AA that you want to cancel everything after DFW, book another ticket, etc. etc. I think you're making it too difficult and/or confusing the AAgents. It really is a simple phone call that should take no more than 3-4 mins
You: Hi, I have an award ticket, PNR XXXXXX
AA: Yes I see that, how can I help you?
You: I just checked online and there is now a nonstop DFW-MIA available, I'd like to change the last 2 segments to that
AA: No problem, we made the change, you should have an email confirmation shortly
If DFW-MIA is in fact available then there's absolutely no need to waste more miles to book it separate or whatever, and there's no risk of losing your QF seat.