I feel for you. We're going through a few things here too. Red has lost weight, has sensitive stomach, doesn't eat the Forti Flora, though it doesn't help 100% anyway (when I force feed it). His bloodwork was ok, but he has smelly diarreha poops. And he's lost weight. Has allergies too and itches, though that's improved.
Ours are older too, and I'm nurturing several of them now for different reasons.
Fuzzy (17 1/2), wants to drink all day and eat, but doesn't do it on his own. I have to syringe him everything (he laps off the end of syringe but won't drink from bowl). He eats LITERALLY contstantly. Every hour at least. He's all skinny.
Whitey has to have special watered A/D to keep his stool soft, so he can poop. Vet said Megacolon, but I don't think so, he wanted him on meds but the watered A/D works fine. He gets that 2x a day and free feeds all the rest.
Red gets special food, and his forti Flora.
Leonard gets watered A/D all day long (maybe 4-5 times). He needs the liquid cause hes in early kidney failure and I won't do subq's. He's actually doing pretty well. Will be 16 in March.
Oh, and Whitey was limping (still is). Vet thinks it was an injury. He game me pred, but that didn't seem to help. It's been about 3 weeks since at vet. If he's not better soon, I'll be taking him back. But he feels 100%, comes hurrying over for certain foods. But then he lays. He lays to eat too.
So it's a constant workload here. Jenny's inside too, so she's seperated in sunroom, needs her special attention.
I hope kitty will feel better soon and gain some weight. i'd be concered with under 6 pounds, my vet seems to think under 6 pounds is in the danger area on a normal size cat. Sounds like you're doing everything you can though. You may want to try deli turkey, etc if that works too.
Let us know how it goes.