Opensea might be verifying the rights owner has attached their image to an NFT, but that still doesn't mean they have transferred any rights to the holder of the token. Correct me if I'm wrong I've not looked up Opensea's T&C's (have several other's).

Also whenever people talk about ownership in regards to copyright, it's a flag that they probably don't know what they are talking about.

For most of these people the NFT is the valuable part. If Twitter verifies your opensea nft and let's you put it on your Twitter profile that is the value.

Wouldn't displaying an NFT where you do not own the underlying image technically be copyright violation?

Technically violations are determined by a court. If no one has accused you of a violation and no court has found you in violation then you are not in violation.

A separate question is would the copyright holder win a lawsuit or DMCA takedown request? I don't know

Ah yes, relying on a single entity for authentication of the purchases in the market.

How very decentralized /s