Air Conditioning - CAC leaking water from plenum

Total amateur here. Had my oil furnace replaced two years ago by the wrong people. I didn't catch on until now (duh!) that they probably caused the following problem with the central AC:

Every time we turn the AC on it leaks water from the plenum at the joint--enough to flood the basement. I've had two professionals look at it. #1 said it probably needed a new coil. #2 said probably needed a whole new system, even though the compressor works fine. A third professional said over the phone that the trouble could be caused by the coil being crooked, and he implied this would be fairly easy for a homeowner to fix. I'd rather not pay a couple hundred bucks and upward if I can fix it myself.

The plenum only has a small door (maybe 5 x 5 inches), which doesn't seem big enough to manipulate the coil through. How hard is it to open up the plenum at that joint? And then how do you straighten out the coil? And put the plenum back together properly?

Thanks!

The problem could be an incorrectly mounted coil, an incorrectly mounted drip pan, an incorrectly mounted drain line or a plugged drain line. The plugged drain line is the most likely.

Opening a Lockformer seam on the plenum would be difficult and re-closing it almost impossible. If the plenum was assembled with "S" cleats and drive clips it might be doable. Some pictures may help. Pictures need to be first uploaded to a photo hosting site such as photobucket.com and the the URL posted here.