Good natural food for sensitive cat + adopting a buddy?

Hoping somebody can help me out. I just lost my dearest cat, Puck- he developed crystals in his urine and was totally blocked and due to financial constraints I coudn't save him (vet bill could hav easily broken $1K if everything went right). He had been living with my mother the past couple years and was eating friskies and 9lives, I switched him to Authority for sensitive skin and stomachs when I got my new place a month and a half ago…. vet said it could have been the food, or the stress of moving, or inadequate water intake (they have an auto waterer that I clean throughly every other day).
His best friend is left behind. Puck and Batman were best buddies since they were kittens. When the Bat was an orphan and little baby puck would clean him up and cuddle him, something that they continued for the next 3+ years….
Batman is the one with a sensitive stomach- . I decided since I don't want to risk another kitty with crystals, I am doing everything I can do to prevent it. I have done a little research and want to transition batman to a really good quality canned diet (have heard that doing canned instead of dry increases their water consumption even if they do actually DRINK less…)
I picked up some Natural Balance canned food- I liked that it was all natural grain free, had cranberry, magnesium content was consistent with a diet suitable to not aggravate any urinary tract issues , didn't contain dairy (batman gets WAY sick from cow milk) ..etc.. and he ate a bit…and proceeded to puke it right back up. same thing with the couple of mouthfuls this morning.
Any suggestions for natural/organic brands of canned catfood that are good for sentitive tummies? I think I'm going to exchange my remaining cans of Natural Balance for Wellness Brand- my sister uses Wellness dry food for her cat with chronic UTI (he's on medication the rest of his life to control the alkalinity) so I'm assuming that it should be safe. Any other suggestions?
My next question is more of a personal thing,,, Puck was the kitty love of my life. He slept on my chest every single night, his head under my chin, his paws on each side of my neck, kneading and purring until he went to sleep. We would play fetch until he was panting. i've had animals i've loved, but never was this connected to another animal. His loss has been devastating. I feel so so SO guilty- maybe if I made more money I could have saved him, maybe if I had noticed he was sick earlier he would have been ok, etc….
I dont' know if I can get another cat. I can afford SOME expensive vet bills (would have happily dropped about $400 to treat him this weekend. I would have been on ramen noodles until the next pay day, but I would have done it, but I couldn't have afforded what it would have likely taken to save my little boy.). My mother has a cat she wants me to take. He's a nice kitty, and he would get better food with me… but we tried him the other night and batman didn't really like him when they lived together, and he just isn't *my* kitty.
I'm not ready for a new cat…. Not right now. But I hate that Batman is sitting alone in my apartment all day with nobody to hang out with. I worry that he's going to become lonely- he has spent every second that I have been home the past few days curled up my lap.
Taking steve would take financial burden off my mother who is financially pretty tight… he is happy and healthy with her (as well as any old feral tom can be… he's neutered, not starving to death, not hit by a car, etc…)
Also, what if the same thing happens to another cat that I take in? I don't know that I could handle the guilt. I can afford routine vet care and food no problem, but heroics just aren't in my budget,

I think Batman would probably get along better if I got a younger kitten or young cat who is maybe a little timid that maybe he could mold into something he wants to live with…. I think that would be easier for him. But there are so many older cats in shelters….
What would you do? Take mom's cat, improve his situation a little, improve my mom's situation a little, but annoy my other cat to no end…
Adopt a cat from a shelter, maybe make batman unhappy, but possibly save a life…
Adopt a kitten, which is against my moral compass since they are so much easier to place than s, but probably a lot easier on my existing kitty….