I will be flying LH to IAD and (possibly) connecting on a separate ticket on UA.

Given that the connection at IAD to UA is fairly tight, I also have a backup flight out of DCA later in the evening. Whether or not I have a chance to make the UA flight might not become clear until I get my luggage at IAD.

Question: if I were to ask LH at check-in to through-tag the bag to my UA connection on the separate ticket (which I know isn't guaranteed, but I'm flying F so they may do it), I'd do immigration and baggage claim at the midfield terminal. If at that point it becomes clear that I can't make my UA connection, could I exit from the midfield terminal to the city with my baggage, after customs but before transit security?

Alternatively, I would not ask LH to through -tag the bag, do immigration and customs at the main terminal, and then see if I can make my UA flight by going landside, dropping my bag at the regular bag drop, clear regular security, and take the train back to midfield. (or make my way to DCA if I can't make the connection at IAD)