[mothered]

I'm at the last stage of the SE. I said I had the kindle in a plastic bag as I feared for my safety. They want me to send the kindle back and they gave me a prepaid usps label. Should I try to say that usps won't accept it because I was talking with the guy and he said that isn't allowed?

The guy (whoever you're referring to) Is Immaterial. He may say one thing, but their policy states another, hence It's the policy you must comply with.

As per their return policy, do USPS In fact accept returns with a leaking battery? If not, there's your answer.

I agree. I had this experience before, and I had to remind the rep that USPS/UPS do refuse to send hazards (in this case leaking batteries).
If the rep still does not comply, ask to speak to a manager and try again.

Due to being new on the job, some reps are Ill-Informed.

As you've mentioned, always request to speak with their senior officer, hence they'll confirm what you're saying Is true and correct.