Hi, I'm so sorry Ziggy has so many problems, but you are in the right place as people on here are amazing. I was told not to give tuna for liver problems as it's a bit rich, but oily fish is probably good (although my vet said not only fish as it's not got enough nutrients to support an old and ill cat). Have you tried (my vet swears by it) those little ringpull cans of (human) sardines in tomato sauce? If he won't eat k/d minced chicken or the pouches as they are, you could try blending either the Hills or the RC chicken renal pouches so it's just able to be licked up. Hills also do a very easily eaten, quite smelly food in their Science range - Senior turkey pate. You could try it and if Ziggy thinks it's horrible, they'll take it back (if you get a sample can or two from your vet). It's quite sticky but smooth, no hard bits. Mine prefer it to the chicken flavour in the same range, which has more minced hard bits. As senior food, it's ok in phosphorus (twice renal food, but 0.6 per 100g which is not bad). Wet food may be easier on him than dry.
The problem with livers being dodgy is that they tend to get sick on ordinary cat food, even ones they enjoy, as the gravy etc is too rich. I hope you find something he likes and that he can tolerate, poor little guy.
