Why does mamma cat have to wean her kitten right now? She will wean when it's time.
I understand you want to get mamma fixed, and I praise you for that very wise decision, but if she's in a room with her baby by herself, she won't get impregnated again. You are providing regular cat food for mamma, and mamma cat will introduce the kitten to regular food as it matures and can handle the change in diet. We don't cut our children off the breast cold-turkey, by nature, so let the mamma cat do her thing in her time. Then when you are sure she is done breast-feeding and the kitten is fine on his own, then take her to get fixed.

I don't mean to sound touchy to you or any catlover, but we humans tend to forget our cats act on instinct, and mother nature does great without us on a lot of issues. If this is mamma cat's only chance for having a kitten, let her nurse until she's ready to wean. I would think she wants that bonding with her baby as much as we cherished it with our own children.
(And yes, I had to quit breast-feeding cold-turkey so I could get back on the Pill. It was painful for me physically and mentally, and my son too. We'd both been better off if others had let me do it my way and not some man's way.) Please, let mamma cat make those decisions by instinct.

And, I loved the advise to rub cats with towels to introduce to new cats. I needed that advise about 3 months ago, but all are getting along fine now.