Urine taken at noon is not "first thing in the morning", and could be more dilute because he's been drinking and eating recently. Try to get a sample when you first get up, if you can. Depending on a cat's diet, their phosphorus may be higher. FF is not very low P. Try to see if he can eat some lower P foods. Don't switch him cold turkey. Try mixing one spoon into the mix and first, then gradually increase the amounts. Kyle's post in #103 contained a link to a document listing CKD food for diabetics. There are more options than what you have tried. And that same list has the answers for a couple of the Tiki Cat After Dark.
Clumped platelets happen all the time, not something to worry about. I swear clumped platelets, causing lower platelet numbers, happen almost 50% of the blood work results I get. This is a quote from what you entered on the Labs tab:
As for other blood work, it's really helpful if you can either bold or change the text red for values that are out of range. I am used to "world" lab values, so something isn't going to jump out at me unless you highlight it.
I agree with the vet to just keep an eye on his weight and maybe an abdominal ultrasound if he keeps losing. Perhaps there is an absorption issue causing him to lose weight that a U/S might detect. A blood test of his B12 might also help - generally the Texas AMU GI panel is a good one.