Not sure if this is the right place to ask.

When the re-run for the Romulan FE begins, will we be able to re-claim the Reman Boff if he isn't already part of our crew? ie if we used him for training another boff, but now want him to join? Or is it a one time thing, like the Borg from STF Kitohmer Accord?

That is whole reason for replay if you do not have one on your crew, mission replay is the time to get one.
if you do not have one during mission replay you will have the opportunity toward the end of the month to get one!