There is a sticky thread at the top of the Cat Health Forum called
Early Stage Kidney Failure in Cat you will find it to be a long read but there is a link to a summary post in the first one. Lots of good tips, info & links in there.
The Azodyl & Epakitin aren't for weight gain/management but for uremia & phos control. It will be mentioned on the label to keep it in the fridge at 4*C for best results or like some of the ones I purchased had a round green sticker on the lid that said 4*C.
I would still think whole is the most effective way to give the Azodyl, though the company is doing trials on it mixed with food
In order to maintain/increase her weight she needs to eat consistently and a good quality protein. Foods such as Wellness, Evo 95%, Merricks Before Grain 96%, Halo, Holistic Select etc will have a higher animal meat protein content than FF since they do not contain corn, wheat gluten etc.
Does she seem to be nauseous before/during/after she eats? Lip licking, droolng, heaving, holding her head over the bowl but not eating etc
Do you have her dish raised up between 2-6 inches off the ground? This will help to reduce nausea by ensuring her head is above her stomach when she eats/drinks keeping the stomach acid in her stomach. I say 2-6" so you can make it where ever is most comfortable for her size. Even if she isn't nauseous this may encourage her to eat more as the angle of her neck would be more comfortable for an older kitty. A phone book, low box, plant stand etc are good, just ensure the dishes can't slip off & scare her while she eats.
You can also encourage more canned food eating by adding a couple of crushed dry krunchies on the top, tiny bits of cooked chicken, sprinkle of parmesan cheese.
Since she is tiny & not eating much right now I would offer her a variety of different brands/flavours from the list to see what she will eat & expand from there. You don't have to restrict her food choices to one brand or flavour, in fact it's better to have a few different ones in case she decides at some point she doesn't like a particular food, then you still have more to choose from.
Let us know what the test results say