Tee nut. Can't help you on the rest of you questions.

Tee Nuts, I think.

If you have a screw that's jammed into one of those things, you may need to pop the whole thing out and replace both.

If you torque on it enough, you'll either break the screw, or the t-nut. Either way, the whole thing will have to be popped out.

We ended up having to get a pair of needle nose pliers to hold the t-nut while unscrewing the bolts. We had two bolts break but the rest came out just fine, it was a pain in the but though.

Those things can be the devil when they're locked up. It's because some of them were bolted in with a thread locking glue. Some solvents may loose them up, but other times you'll be cutting them off. It sucks.

If the tee nut is spinning in the wood you can take a medium straight blade screw driver and tap the spikes deeper into the wood so they bite in like they are sapossed to do. If that don't work use needle nose vise grips or pliers like stated above.

If you don't want to break the bolt, my suggestion would be to knock it down so the 'T' nut comes out from under the playfield and spray some anti-sieze spray in the thread area and leave it a few hours and see if that un-locks it... If it does, tap the 'T' nut back in place (after bending the spikes back out if they got flattened like the one in the pic) and you're done... it might be necessary to knock the 'T' nut in at a different angle so that the spikes go into virgin wood to get a better grip while tightening the bolt back in (depends of the original holes in the wood got damaged when the nut came out!)... The problem is that populatred playfields don't like to be hammered, so if it's a populated playfield you're working on, maybe press it in as hard as you can, screw the bolt in, and then use the bolt as a pulling tool from the other side to pull the spikes into the wood... Far easiest is to simply break the bolt, knock the 'T' nut out and replace them though... not easy if you don't have replacements of course!!

These are call - PITA nuts, also known as T Nuts

The only ones I have ever had an issue with were on my BK2K. Those were all threadlocked. Man, what a effort it was to remove them. I pretty much had to cut or break off the head of the screw and push it thru the underside of the playfield.