One of the strainers in my kitchen sink (the side without the garbage disposal) started leaking in the putty/gasket area. After seeing a bunch of videos about how to replace it, I attempted to do the same. However, while all videos talk about removing the lock nut and having that be the hardest part, I went and got a lock nut wrench and prepared to remove the old part. To my surprise, my existing strainer doesn't have a lock nut at all, and there are no threads other than the one for the coupling nut at the very bottom. There are 3 thumb screws that appear to hold the strainer in place. The problem is that the thumb screws go through a metal ring, and while the metal ring has slots for what could be a screwdriver, that metal ring won't budge at all. After removing the thumb screws, it is possible to move the strainer, but only as far as it goes until it reaches that metal ring. See pictures below. You can even see that the metal ring isn't exactly straight, but all my attempts to get it to budge (with vinegar, with WD-40, etc.) have not been even the slightest bit successful.
Any ideas, short of trying to cut the metal ring with a hacksaw? It would be a bit difficult being how close it is to the sink.