Should I focus on studying opening theory and the plans of the openings I play or continue with tactics until say 2000 on tactics trainer?
You can go to this forum
http://www.chess.com/forum/view/general/improvement-in-chess-according-to-elo
You can spend some minutes reading the advice and then you can follow the links for the chesspersonality test and the other articles regarding improvement. The important thing is to analyze your OTB games and find what you need to practise more.
which by the way is another contradiction to people who say that openings aren't important seeing that it leads into the middle game).
Although it's true that the opening does lead to the middlegame, it doesn't necessarily mean that you should begin studying with the opening. In other words, the opening is just the series of moves that develop your pieces until you reach a position where you're ready to execute your plan. You should have a plan in mind ever since move 1, but the plan you had in move one isn't necessarily going to be the same plan in move 20. Studying openings is probably the most fun to study, but that doesn't mean it's the most important part of the game.. For a Super GM, all parts are equally important, but for almost everyone under 2000 games are decided by who understands the position better, not by who plays the opening the best. The opening may give you an edge entering the middlegame, but that doesn't matter if you don't know how to procede. Therefore most games below 2000 are decided in the middle game. Now, in my utmost opinion having a specific opening isn't of great importance until you reach a certain level. Consider GM Bryan Smith, I read in one of his articles here that for a big period of time he didn't know whether to play the Sicilian Dragon or the French Defence as Black. (He was an IM at the time he stated that) Therefore if even Titled Players struggle chosing a proper opening to stick with, then it'd be most proper to delay the opening stage until you have a better understanding of the strategies you are able and feel comfortable to create.