So my question is, if I come back to play with him, and I don't have the abbysea collections, ( I only have WOTG, TOAU) when I'm not playing with him will I be able to level up anywhere, and are there still people doing WOTG missions, and TOAU missions? Are there still people partying in TOAU areas, or even maybe some new WOTG areas?
You can level up. People party down in the sewers under sandoria, can get to cap down there. You can also solo campaign (sometimes others will be there) and get exp that way... missions etc are a bit more annoying to get people for but people do, in fact, do them. Realistically, anything that doesn't require three people to enter (assault/salvage/meeble etc) you can probably duo.
However, I think you're missing out on some of the most fun content available by not having any abyssea. You will also be missing some key stepping stone gear (AF3 +1/+2 is quite decent and a lot of it is fairly easy to get). I am not sure if you can do voidwatch without abyssea add ons, I think you can but not sure....
Anyway, given that the ultimate collection was just for sale for $5 keep an open mind about getting all the add ons. Also, there is a new expansion coming in march, including two new jobs, Geomancer and Rune Fencer... don't know if you know that... so it may be worthwhile to wait until it comes out and just get the ultimate edition with all the add ons at that time.