After scanning a few of your games in your archive it would be best for you to concentrate on building 2 of the 5 visualization memory banks that you must build in order to increase your chess playing strength.
1.Tactics visualization pattern memory bank
2.Mating Net visualization pattern memory, bank
No, you shouldn't really study opening theory. Study opening principles instead and the other things you covered. Until you get to higher levels (closing in on master level probably) you don't need to worry about opening theory as much.
Like you mentioned, most players your level don't know it either and you'll be out of theory/book pretty quickly and on your own where the other skills will give better benefit.
yes, they are useful to know, but study other things first like endgames. for me, if i lose in the opening i lose the game.
You need to know basic opening principles. Study some basic opening concepts. You don't want to fall into opening traps.
I wonder if I should start studying openings. I've played for an year in theory, but in practice the real time I studied and played is a lot less. Most of my opponents don't know opening theory, both OTB and online, so I think it's useless study opening theory now, but instead study other things like endgames, tactics, positional play, commented games, etc... Plus in the '800-'900 opening theory was little but they played wonderful games. I'll buy Reti's classic "Masters of the chessboard", which contains a bit of opening theory, do you think that should be enough for me?
p.s. Don't look at my rating, I played mostly blitz and I lost my online turn based games due to time or resigning because I can't play at the moment.