IB is great on Lapras but mostly because it is a dual Off/def move. With defense what it is now I would not risk getting HP and losing an attacker for the chance.
You are just right IMO that you should let the end of the Moltres availability play out (I am doing the same right now and it is painful lol) but then more chances won't be available until who knows when. If you want to take a couple shots that is up to you. Other mons you at least have a chance to catch/raid/evo for better movesets. We aren't sure what the availability of these legendaries will be post event.
Edit to add: I know it is hard to wait, I struggle with such impatience in the face of mons I really like and want to power/TM too. The very best course would be to wait until Zapados makes his run too and after that see where you are. Maybe you get 2-3 super incredible Zaps who need the same resources and are more immediately useful due to the meta?