Buying now will give you access to the beta, if you want to check it out, and the full game (beta saves will not be compatible after release). There is no separate beta key, the Early Access version should just become the full release.
As Grodit said, the golden grail and trunks will only be available on release; they are not in the beta.
Larian has the physical boxes in their office now, and will be shipping them out starting next week (if they have not started already). Release is in 10 days, with no chance at all of the game being cancelled.
Combat is turn based, exploration is real time. In co-op one person can be trading while the other is off in combat, or there can be 2 separate combat instances, etc.
