Thanks for your detailed explanation "br",
but actually the problem isn't so much inside the thumb opening. I can get my thumb into it easily. And I've checked the knot, it isn't too tight. It's the back of my thumb that hurts when I catch the ball and try to close the glove around the ball. If you take a look at the very first photo that I posted, you can see the sort of drop-shaped thing that is inside the thumb. That thing is probably made of plastic or PVC, and I've sadly come to the conclusion that there's nothing I can do about it. If I want to use that glove, I'll have to lose my thumb...
Thanks anyway. I'm now working on a poor old and tired catcher's mitt. For now it's a ruin, but wait till I'm done. I'll post other photos. You're absolutely right, this is a great feeling to bring some life back to an glove. This is one of the things that I particularly like about baseball. There's so much you can do on your own, it's like re-inventing your own game at home. I love that.