i do not add loctite and reuse the same gaskets. I have spare gaskets just in case the originals tear but they still look brand new.
it’ll be pretty obvious about the injector as well. It’ll be in the rail and not the head. I’ve never had one come out but always check and apply oil to orings before sliding rail back tight. You should only need to slide the rail back on one side. I forget which side though. Sorry.