OK, JUST STOP! RIGHT NOW!
This girl has come on the forum asking for advice on how to take care of her cat and her soon to be newborn kittens and all you guys have done so far (most of you) is lambaste her about allowing the cat to get pregnant and then about not aborting the kittens by having the cat spayed. Just stop it. Give this person some support, not criticism.
First of all, you said on the other forum she might have gotten pregnant sometime in mid December and she was half way through the pregnancy when you first posted per your vet. That means she could deliver as soon as the next ten days. Do use the diatomacious earth to dust her, the house and her areas to decrease the flea load in the house. DO NOT use a spray chemical as this toxin will get into her system and harm the kittens. The only reason chemical sprays kill bugs and not people or dogs and cats is because the bugs are so much smaller physically than the mammals involved and need a much smaller dose to be lethal. It is still a toxin and it can and does get into mammal bodies when it is used.
You will be able to tell when she is ready to deliver because she will produce a clear to cloudy exudate you can express from her nipples. This is the colostrum all mammals produce before the first milk to boost the immune system of their babies. She will also be nesty, looking for places to hide to have her babies and she may want to cuddle a lot and lie around purring like a freight train. You will not be able to spay your cat until all the kittens are weaned; a minimum of ten weeks after their birth. Once you have them eating soft foods (either soaked dry food or canned food), you can separate the mom from the kittens and dry her milk up in preparation for spay surgery. Two weeks after removing the kittens should do it. Of course, I never separated the kittens and mom and let her nurse as long as she wanted. Miri nursed her male kitten for five months. But I was smart and had the Siamese who impregnanted Miri castrated while the kittens were still very small so Miri could not get pregnant again before I could spay her.
No, I do not think your vet is trying to get you to unknowingly kill the kittens by giving your cat wormer and flea spot. She was trying to keep the kittens from getting worms and fleas from the mother herself. It is one reason why so many feral newborns die within a week after birth; they pick up the worm eggs off their mother's fur, hatch almost immediately and suck the nutrition of the the kittens, and the fleas literally suck the kittens dry of blood and kill them.
I do understand how difficult the decision is to spay a pregnant female. My Maine Coon queen got pregnant by my Siamese tom accidently. Miri had four kittens, three females and a male. At the age of 10 weeks, three days before they were all to be neutered, the girls all went into heat and their brother happily mated with each and every one of them. So. I had to decide whether to abort the kittens with the spay procedure in three days or let them birth their babies. I already had 23 cats and really could not afford to care for more, so I reluctantly decided to neuter them. I have always wondered what their babies would have looked like. But the potential increase in my cat pride by twelve more kittens.... I just could not do that to the family.
What other questions do you have? Sorry I got to the forum a little late. I understand you must do what you must do with birthing the kittens. Your choice, your responsibility. Good luck with her.