I noticed that the shell has a pretty severe dent behind/underneath where the strainer was.
There are two different things we could be talking about:
(1) The snare beds are crimped and many people mistakenly think that's a big ol' dent in the shell.
(2) Sometimes metal shells do get dented, especially under pieces of hardware that stick out and can get banged (strainers, lugs, etc.). The Zoomatic is particularly prone to this since the shaft of the adjustment knob can concentrate stress on a very small area of the shell.
If you really do have a dent, it's usually no big deal. With the right tools (which can be pretty easy to improvise), removing the dent is a piece of cake. I've got a piece of wooden 14" shell scrap that I use as a mandrel since it's got the same curve that the shell's supposed to have. Remove hardware from the shell, place the shell on a workbench or hard surface with the dent facing down, place the shell scrap on the dent from the inside, and gently hammer the inside of the scrap to tap the dent back into shape.
There's another issue and that's the Zoomatic strainer doesn't seem to work all that well. The knob on the front of the strainer that tightens the snares seems stripped, so the snare wires are always really loose.
Sounds like the shell nut could be stripped. It's the threaded nut attached to the shell that the Zoomatic's adjustment shaft screws into. You can get a replacement from jp2creations.com for $25. (Go to jp2creations.com, select Parts in the left column, click Slingerland Zoomatic Strainer, and you'll see the shell nut at the top.)
If the shell was banged really hard on the strainer, or if someone overtightened the strainer, that could explain both a dented shell and stripped shell nut.