Yes, the drain in the wall should be lower than the disposer. The only way you can fix that is to cut out the wall, cut the Drain, waste and vent pipe off and lower the Tee in the wall. Its not such a big deal with plastic pipe, hoping the pipe inside the wall is also plastic. You can cut the pipe by wrapping a cotton string around it and pulling it back and forth rapidly (nylon string will melt); which makes a tight place a lot easier to cut.
You will need 3 couplings to hook it back up: one under the tee and two to put the piece of pipe back in above the tee. Or you may use one new tee and one coupling to put it back (tees are aprox $3 while couplings are approx $1) which would make a much more attractive job. More than one coupling in sight looks like a botched job to me.
You may not be able to move the vent pipe up far enough to get a new coupling in the line; in that case you can use a stainless steel wrapped, neoprene slip-joint coupling. It has 2 hose clamps on it and can be slid up the vent pipe until you get the lower pipe in line, then slide it down and tighten the 2 clamps (its called a stainless steel fernco clamp).
I'm proud of you for knowing you need a P-trap between the drain and the wall. You can also look around at The Depot or Lowes and find a Continous waste pipe set for hooking up the sink and disposer. BTW, the disposer has a plastic plug in the dishwasher hose connector that you will need to break out with a screwdriver before you connect the hose.
I know, it sounds like a major undertaking but give yourself time and take a break now and then to gather your thoughts and plans and you'll be just fine.