Can I re-enlist for a duty station if I am “on assignment” but with no official orders yet?

No, once you're on assignment you can only reenlist needs of the Army. Being on assignment constitutes the beginning of the orders process. Receiving your orders is the final step in the process, and also on you because you need to process your levy brief prior to getting your physical orders.

Only thing that can help you now would be to try and go somewhere that is a higher priority assignment. For example, I had orders to Ft Stewart. The SFAB was a higher priority assignment, I'm not at Stewart.

Odd that Retention couldn't help. One would think that if you re-enlisted with Choice of Assignment then that should negate the previously given assignment. Especially if the orders for the chosen assignment come in before the other ones.