You said she poops every other day, which means things are moving at least. What are the poops like? Runny, hard, small, large, black? That could help determine what the issue could be. If they seem to be normal like they should be then maybe she just doesn't go everyday. I'd suggest giving her canned pumpkin. It's very good for their stools no matter what the issue is.
The "barrell-ness" could mean she's very backed up and can only be fixed by a vet but it could also mean she's just fat or has worms. Has she been wormed?
I have no tips get the miralax down, sorry. But I had a cat that would only drink running water, like out of the faucet. She refused to drink from a bowl no matter how often I changed it.
If she does need surgery I hope you will at least try to find a rescue group that might take her. Many of them won't, but some do.
Bless you for taking her in and I'm sorry it's not going well. I have no animal expertise, only experience.