Firstly well done on helping this little kitty it looks like you are really set out to make a little home for it.

Now down to the point.

take the flea collar off. this could be the cause of not only the oiliness you speak of but the diarrhea too. most flea collars are toxic and a waste of your money anyway as they only repel and don't do as they really say on the box or the fine print will tell you different.

Tea Tree oil is ok if diluted in a bath but not so much as a topical, basically don't let it go where kitty can lick it or get it on their paws and lick it. they make pet shampoos for cats and dogs with tea tree in them. I would get some ointment from the vets for her ears personally.

As for the diarrhea, what did you worm her with? you may need to pick up a stronger wormer for her (Milbemax or Profender) from the vets as she has been exposed to a lot of nasties you also should pick up a proper flea treatment for her as well such as Stronghold or Advocate these will help with not only the fleas but with the roundworms and mites in her ears. You can pick a prescription up for these at any local vets and either buy them online with the prescription or take them to your pharmacy for them to order for you. Or you can just purchase them from your vets but they might be a bit more expensive.

Hills kitten food is a lovely food for her, my suggestion would be to keep her on it and not change the diet until you have treated her with the above mentioned and see if the runs subside.

I think really your best plan should be to get her seen by a different vet. If she is quite elderly there could be underlying problems causing the diarrhea and you may have to let her go but try to keep positive and if you feel she's in pain we're all here to support you

all the best.