Papaya does not have a pit, it has seeds.
I give my chins dried papaya all the time. It is good for them and gives them certain vitamins they need. BUT, maybe he has developed an allergy to papaya. Check in the cat section of the pet store, they have stuff there (Petra Malt I believe it is) that is for hair balls, it's in a tube, just give him a small amount (about the size of a pea or smaller) about once a month to rid his system of hairballs.
Peaka is definitely a "senior citizen chin" and his system may not be able to tolerate things he has had before. Chinchillas and chinchilla's bodies are notorious for disliking changes of any kind. At his age, this is especially true.
I would definitely stay with the hay that he seems to tolerate well and never buy the other one again.
I have found in my older chins that their systems do tend to react to sweets more, so you might want to try to cut down on the size of the treat rather than the treat it's self. If he likes his treats, it might upset him if you take them away completely. Being up-set can also cause a chin to have diarrhea. (I would stay away from the papaya since you have already tried giving him a small amount of it.)
Cranberry juice is good when there is a urinary infection (dark colored or strong odor), it helps to keep the urine flowing and flush out the bacteria for a chinchilla as well as a human.
I've never used cranberry juice for a bowel problem.
You have to remember his age and think about how his system has changed over the years. He would be in his 90s if he were human.
If his poos get mushy again and/or is foul smelling, put him on a solid non-porous surface. Have a magnifying glass handy. As soon as he drops a piece of poop ... do not pick it up or move it ... look at it under the magnifying glass. Is there mucous covering any part of the poop? If there is, he needs to see a vet a.s.a.p. Ask the vet to run a test for Giardia.
http://www.geocities.com/jobernstein1949/ Read the article on Giardia. More than likely it is not Giardia, but just old age ... but it is best to be safe and not sorry.
Why did you separate Pooka and Peaka? Are they still separated?
Jo Ann