I tried screwing back the original lens but it was very tight as well. Anyway, seller agreed to send replacement holders.You need a bit of force (after making sure that you are not cross-threaded). The set screw appears to not go through the mount but rather "pinches" the lens in place. I think that causes the inside threaded part to deform a little bit, resulting the resistance you are feeling so you need to get the lens past that point and it should be easier to screw in the rest of the way.
Even the lens A2 metal cap came off when I tried to remove the original lens.When you say screw it in a bit, what does that mean. How many turns (or less) of the lens before it stops, this will give a more realistic idea of how much the lens is being screwed in.
It should be snug, but you should not have to force it. The whole mount assembly is not rugged enough to be forcing the lens to screw in.