Thanks, everyone, for the hand-holding and the expert advice! We've made a few changes in the tank: the right kind of salt, for one thing, and we also removed the too-small and way-too-large extra shells and replaced them with half a dozen shells that are only slightly larger than the ones Jeff and Jerry were wearing when we acquired them...and guess what? Less than 5 minutes after we added the new spare shells to their environment, THEY SWITCHED SHELLS!! We actually got to see it happen--and I know y'all won't think I'm nuts when I tell you it was SO DOGGONE EXCITING!!
After that, they had a bit of a dominance display, no shoving this time but some posturing and raring up on walker claws, and even some poking with pincher claws--but there was no actual pinching or grabbing or pulling, and the whole thing was very short-lived. In the end, they sort of edged away from each other, and then they went to explore some more spare shells and eat some peanut butter--and dip their claws into their new-and-improved salt water. This time I knew not to panic at the dominance display, and their behavior in general also seems more...normal?
So now that they seem more settled, I know the next thing to take care of is that too-shallow substrate. Does it seem like a good plan to add more tomorrow? That will mean removing the crabs, dishes, etc. for a few minutes, but if we get everything prepared ahead of time the crabs shouldn't be displaced for more than about 15 minutes. Too much stress? Or is deepening the substrate urgent enough to chance it?
As soon as this question is resolved, this thread can be closed. Again, thank you, everybody.
I'm not sure when that recommendation started. It's not a bad one since it makes it easier to maintain the substrate, but there's no danger in having a full height UTH, it's just often not needed. If the tank is in a cold spot though, that extra heat can make the difference.
Thanks, all.