Level sagging floor joist
Unfortunately it is very confusing, at least to me. A good step would be to post a few pictures. Even better would be a scaled diagram showing the framing of the joists and the beam (the beam is the doubled 2x8 you mentioned a couple of times. As for fixing sagging framing, that topic has been discussed many times on this forum, and it would be worth your while to do a search and read some of the threads, there are many recommendations on how to measure the sag, how to fix the sag, and how to prepare the subfloor.