I think I damaged by BB and want to make sure I understand what I have before I try to fix it.

The problem is, the BB was loose, so I tried to tighten it with a Pedro's tool that didn't fit well, and I mashed part of it when the tool slipped. Now the seal facing (?) isn't completely flush at the mash point and there seems to be some rough points in spindle rotation.

The BB is marked, but I can't figure out exactly what it is to order the same BB. On the exposed metal it says:

"MegaExo BC 1.37"x24T FSA BB-8000/6000"

The seal face says: "FSA MegaExo [diameter] 24mm MS185 . . . V4.9"

The shell width measures 68mm. Crankset is FSA Gossamer (2 chainrings) but I can't figure out what specific model.

So . . .

1) If I understand, my frame will accept any English-threaded BB for a 68mm shell and compatible crankset. Is that right?

2) All FSA MegaExo BBs are compatible with all FSA Gossamer cranksets (right?). Can I buy any MegaExo BB as a replacement?

3) Will other BBs work with this crankset? I have an REI dividend . . . they have some Shimano and Truativ BBs I can get for no actual cost.

I found a Park BBT-19 that will replace the Pedro's.

Sounds like you may have cross threaded one of the BB cups, with out pictures this is hard to confirm, would be worth taking it to a good LBS, and getting them to chase the threads.

For your questions

1- all road bike BB's are 68mm (going on you have a modern frame, and it's not got a Italian BB thread)

2 - As long as the crank is for an HT2 type BB then yes.

3 - MegaExo and Shimano HT2 are compatiable, Truvativ aren't

For a tool, would look at a Park BBT-9, more user friendly than the workshop BBT-19, and has a preload tool built in, which is useful if you need to work on Shimano HT2 cranks https://www.parktool.com/product/bott...ket-tool-bbt-9

Thanks much.

The cups actually have never been removed, only tightened a couple of times. I'm certain they aren't cross threaded, at least not by me. The damage I did was to mash in the outside edge of the cup in by ~1mm or so.

I've come close to doing this myself, and I've seen it done by a few home mechanics.

While using a torque wrench on these crank bolts is advisable, it's not absolutely necessary. Just remember that these bearings need only enough side load to prevent the spindle from shifting from side to side. Finger-strength tightening is enough, and if it's a little too loose, you won't break anything by riding home and tightening it up a bit.