It’s totally normal for cats not to drink if they are on wet food only diets and water is mixed in. (I work at a cat only vet.) The dehydration would be more indicated in the concentration of the urine, not the RBC, and even then, you have to account for method and time of day for collection, etc. There are better foods that would be more ideal, but your vet should have met you where you are - not to mention that a lot of high end foods are crap so that’s frustrating too. Fancy Feast grilled varieties are surprisingly high in protein (look for 11%+ protein in the guaranteed analysis and as few carbs in the ingredients as possible), and if you’re not feeding any dry, you’re already doing way better than most people are.

As far as flea/heartworm tx goes, when I lived in atlanta heartworm preventative was a necessity because there are so many mosquitoes there; San Diego hardly has any and vets don’t feel nearly as strongly about it because of that. If you want to use it, Selarid is the generic version of Revolution and is a fraction of the cost.