Just to start out, the quickfire PCB will not work with a phantom plate unless you take a dremel to the plate. The quickfire and filco controllers sit on top of the PCB and will interfere with the plate unless you make a cutout for it. Also, as I have a phantom plate and PCB in a quickfire case, you need to make a few modifications to the case as well.
now as to the other questions:
1) I know filco and coolermaster quickfire TKL cases work, those are the standard ones. It appears a keycool TKL case would work as well, and I remember reading somewhere that ducky TKL cases should as well, but I don't have one to check and make sure. Also all these products have a metal plate that has a
90° bent lip at the top and bottom, so to get a flat metal plate to be supported on all sides you need to put in
thin rectangular spacers at the top and bottom on the case, if not it is just supported by the standoffs in the case and the sides of the case, which is sufficient, but does lead to a little bit of flex in the upper corners and top function row (I dont notice it much, but on tight fitting stems it can be a bit irritating when pushing the caps on).
2)
the phantom PCB of course, though otherwise I don't know of many aftermarket PCB's with the correct spacing between the main block and the function row, most korean PCBs have a smaller gap.
3) yes, the phantom plate supports plate mount cherry stabilizers
4) yes, with one covet, like on the other cases, you need
spacers to make sure the board sits correctly in the case if you are using a plate that does not have the bent lip. And if you are using a phantom PCB, since it uses a teensy mounted on the bottom as a controller, a milled out section in the bottom of the case to prevent interference with the teensy would be needed. Any other PCB should work, as long as it has the same clearances a filco PCB would have in comparison (so no guarantees there..).
If I were you I would check out your ducky board and see where the controller is on it and mod that one, though since you are getting rid of the plate as well the leopold might work as a donor PCB and case as well if the controller is on the bottom (same for the kul as well). Though for dimensions I have a phantom plate here, and they measure ~347x127mm, which matches a quickfire plate I have sitting here as well, so if you break down one of those boards you have and compare against those measurements it can give you a partial idea of what might and what might not work.
And when searching for what the keycool PCB might look like I ran across this on aliexpress:
87 key layout kitso with the kit you could theoretically just buy a phantom plate and then hit the ground running. But I would use what you had at hand first as the keycool possibly has different plate dimensions, though it is not likely.
