Without too much trouble or hiring a lawyer, you are probably looking at a $500 to $1,000 travel voucher, or perhaps a system wide upgrade. Your most sailiant points about compensation are in order of importance:
1. When the pplane landed in the deversion station, the carrier did not quickly accommodate pax with hotel and meal vouchers. Instead they made you wait, etc. While they may not have operations at this station, certainly if they paid enough, they could find someone in that city to act as their agent. I think this failure is your key bone of contention;
2. The 3-4 hour wait on the ground in FRA with little to no announcements;
3. You could indicate that it seemed UA was rushing to get the plane out; and
4. Indicate the terror you felt, that you thought your number was up, etc.

For better or worse much of what a company does to compensate someone is assign a $ value for the inconvenience or problem, etc. Given that some overnight bumps would yield a $1000 TC, I don't think this is unreasonable. Again not so much because they had a mechanical, but because from the reports once the plane was on the ground they kind of continued with a run around with the pax probably due to corporate cost cutting. E.g. not wanting to give out hotel vouchers. Go on about how this really enhanced your stress, etc. It sounds like AA handled it better in the above narrative from AA that was reported.