"when the npub is added, bitpopart said it says something like, can't access nostr dms due to lack of access to pivate key."

Correct this isn't an error its just a message letting you know it can't read your DMs. As for not being able to be added I don't know about that. As for trying to redeem in Amethyst if you have your LN address set as npub@cashu.me in the redeem button won't work because its trying to redeem to that LN address which would just mint a new cashu.