Junction box questions

Hello! In my attic, I have a bunch of older NM cable that was spliced together not in junction boxes. I would like to tidy it up and put those junctions in boxes. Since it may be NM w/o ground, I wouldn't have an easy way to ground a metal junction box. Is it permissible by code to use a plastic junction box in the attic space? I'm also wondering about using a 1-gang box to make a connection to extend some NM-B elsewhere. It currently runs to a poorly located outlet. I know j-boxes have to remain accessible so I can't use a traditional square box behind the wall there. I would like to use the existing 1-gang box, remove the receptacle, and just make the connection in there. Then, I could cover it up with a blank plate to leave the junction accessible. OK by code? Thanks all!