I've got about 500 rounds through my hellcat so far, and both my 11 and 13 round mags are HARD to load. I use an Uplula every time, and have never seen dents like that. I can't even think of what, in a mag, could DO that to a round. Those don't line up with any part of a mag, moving or otherwise. Does the mag in question, have a crease or a dent down one or both sides? If it does, that could potentially explain both the dented rounds and the sticking spring plate.
Agree on the disassemble-and-check, though. Also, if you press on the spring plate with a screwdriver, can you drive it down as if it were a loaded mag, and when you release, does it spring back up?