You do not need your Platinum Card to use your Priority Pass membership. The lounge staff has no knowledge of, nor interest in, how you obtained your Priority Pass membership, or how you will be billed for the visit. They merely record your entry and if you were accompanied by any guests.
That's right. Priority Pass, however, does look for irregular usage and has been known to cancel memberships; not of people like the OP, but limo drivers and airport employees, etc.