Every table should have a primary key. Otherwise it is not a relation. Neither seam-gen nor Hibernate nor JPA will ever support tables with no primary keys. I have no idea where this table comes from.
Its a database view, not a Table, and many applications have database views which do not have primary keys.
(1) probably some built in view of your database
(2) do not know. you would need to show the full error message/stack trace for me to see what is causing it
(3) views (or tables) without a primarykey is currently handled by letting all columns be the composite-id. That gives problems for seam-gen pre seam 1.1.6, but is fixed in cvs so you should get better mileage there.
That being said I think we need to start ignoring/skipping table/views without a proper identification.