Will they accept a surrender of a cat with diabetes without putting him down?

That I don't know

Try another animal rescue or contact PA SPCA in Philadelphia to surrender your cat. They will not kill him / her, and will try to find a home or keep them for their lifetime. LC Humane will destroy the cat.

From what they told me previously, they only euthanize when the animal is beyond saving, or if they have attacked a person. So, I'm sure it should be fine if it just has diabetes, but if you are still concerned I would call them, they are very helpful.