We are talking about a bar, even if it is at a nice restaurant.
I wouldn't have any issues with asking if B/C could shift down. It's more informal at the bar, anyway.
They don't both have to move- B could take D and C stays seated. That's just my engineering brain talking- in reality I think it's more likely that they'd both move.

If I wanted them to make the offer, I'd seat my gal at A and stand next to her while we ordered drinks. This gives them the option to move away from the not-so-nice end of the bar.
If I didn't want to push the issue and they didn't make the offer, I'd rely on my charm and charisma to make the hour-long-wait-for-a-table go by in an instant.