I feed P and S wet food (Whiskas supermeat, chicken variety - they don't like the other flavours or makes) but sprinkle about half a dozen Go Cat on each dish. If I don't put the dry food on top, they go on munchie strike; especially S! But their dry food is strictly limited.
A cat can live happily without teeth - Polly hasn't got any! She sat on my lap one day, years ago, yawned, and I noticed that she'd lost them all. I'm not sure if she had any when I took her in. She manages perfectly well. But if her kidneys went, or she got a serious bladder/kidney infection, I'm not sure she'd survive at her age.
A way to check if puss is dehydrated: pick up the scruff, hold it a couple of seconds, let it go. If it springs back then puss is okay, but if it remains standing, or takes a while to go back down, your cat is dehydrated. This was taught to me by a vet I used to date.