Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your cat may have an open pyometra.
A pyometra is a uterine infection.
I suppose it is theoretically possible that the infection could only be in one horn (side) of her uterus, but if her cervix is open such that the infection is draining that should lead to her losing the kittens even if they are in the other horn.
The other concern is that the toxins from the infection and the bacteria that caused the infection will be in her bloodstream and spread to her other uterine horn and kittens in that horn, and I would expect those things to be fatal for them.
I understand that she is on antibiotics, and those too can be detrimental to the kittens.
In short I don't believe that she will be able to carry healthy kittens to term with an open pyometra.
If you are waiting to for her to deliver kittens and then have surgery I would not recommend that. She may get much sicker, and I don't believe that the kittens can be saved.
Please let me know if you have any further questions.