I have two of those. First, find a different vet, jeez that ones sounds awful. They shoudl at least be willing to look up stuff and learn to help you.
I haven't used ex-lax, not sure how it is for cats.
Mira lax, maybe about a quarter teaspoon for a kitten, twice daily should help a lot. There are pet-safe enema, but do NOT give a human enema, if she hasn't pooped for 3 days. Giving SQ fluids is also very helpful, and your vet should have thought of that.
I would call around to different vet offices in your area, explain that you have these kittens and are unhappy with your current vet, you need someone who is willing to help them with constipation and incontinence problems.
Owners of special needs pets often have to stand up for their pets, it's kind of sad but it's true.
you can try expressing her bladder, sometimes when they are constipated that also makes it difficult for them to pee, not always but sometimes.
that is very very young for FIP. FIP generally occurs around 7 months old, also it is almost unheard of in outdoor cats. However, many vets who don't know what is going on will say 'FIP' rather than put time and effort into finding out what is going on. Get away from that vet, fast. I don't think she/he is committed to really helping a special needs kitten