Can we do a get for delete marker? And Can lock and retention be applied on the delete marker?

Hello.

Can we do a get for delete marker?

Do you mean you want to download the deletion marker?
If so, the deletion marker cannot be downloaded.
https://docs.aws.amazon.com/AmazonS3/latest/userguide/DeleteMarker.html

If you issue a GET request for a delete marker, the GET request doesn't retrieve anything because a delete marker has no data. Specifically, when your GET request doesn't specify a versionId, you get a 404 (Not Found) error.

Can lock and retention be applied on the delete marker?

Even if object lock is enabled, the deletion marker itself can be deleted.
https://docs.aws.amazon.com/AmazonS3/latest/userguide/object-lock-managing.html#object-lock-managing-delete-markers

Delete markers are not WORM-protected, regardless of any retention period or legal hold in place on the underlying object.

1. Can we do a get for delete marker? Delete marker doesn't have any data associated with it, so if you run a GET request you will get a 404 Not Found.

2. Can lock and retention be applied on the delete marker? No you can still delete the Delete Markers if Object Lock is enabled.