The time to command after the merger won't get longer. I would suggest there will be plently of opportunities for the F/O's. I believe ThomsonFly have been promoting plenty of their people over the last couple of years. However, keep in mind the retirement age has recently gone up to 65.
If the merger doesnt go through, FCA will not be merging with TCX/MYT. If FCA/TUI is blocked why would FCA then merge (be allowed to) with TXC/MYT? Doesnt make any sense.
8 years is a long time in aviation, plenty will change.