I didn't take the 4.3 via OTA, I upgraded to it using the file on my sd card, on the unrooted phone. I do have TWRP recovery on it, and it does not brick my phone. My rom is stock 4.3, which is a TW rom. So, I don't think any of that has anything to do with my microSD card problem.
I do have the proper Samsung USB drivers, and I did try using several different sd cards. I did not try a different cable, as there were times I could see the sd card over the cable, but the card had to be reported as mounted on my phone in order for that to happen. And my phone keeps unmounting the card randomly.
I think you are correct in that this appears to be a hardware issue. I took the back cover off my phone and left it off, and the card stays mounted. However, as soon as you press the sd card area on the back of the phone (I just put my finger over it), the sd card becomes unmounted. The reverse is NOT true though--you can't just press that area to get it to mount again. It appears that, for an sd card to remain mounted on this phone, I can't have battery cover on it, and can't have anything pressing on that area. Yeah, that's not exactly a realistic expectation. It's a defect. This puppy needs to be replaced. Thanks for helping me rule out "user" issues.
