Before you panic, I would have everything looked at again and retightened according to spec. You indicated that all of the screws seems to be backing out and that the transmission seemed to be put back together poorly.
With that said, take your Sentra back to your dealer and have one of his transmission specialists tighten each part of the transmission to spec with a torque wrench, not with an airgun.
Many times when technicians are in a hurry to get a job done so they can move on to another job, making a double fee if they are quick enough, they will use an airgun and wrench to torque things down. This automatically means the device is over torqued. An over-torqued transmission is going to perform poorly simply because it is too tight and parts that are supposed to be in one position — and would be if they were properly tightened to tolerance — are out of position. This means that you might end up with a skewed transmission.
Now I know that the dealer’s technician will likely complain because using a hand torque wrench and looking for the proper tolerances takes extra time. In the long run, I think it is worth it.
One other thing to have rechecked is the pressure plate assembly. That assembly is made up of not only the pressure plate but also the throw-out bearing as well as the friction plate and the flywheel facing. (When you press the clutch in, the throw-out bearing separates the pressure and friction plates so that you can shift your car without having the engine and drivetrain connected.
I am willing to bet you that your technician will find something improper about the repairs that were made. This could cost you as much as $800 — yes it is expensive — but it beats spending $2,750 for a new transmission and parts, doesn’t it?