I'm also on the RT-AC66U, although with Asus firmware rather than Merlin; haven't had a need to go non-stock on this one yet. Here's a screenshot of my wireless tab:
The dropdown for Band works as you'd expect, as well. I can also confirm that the client list is working as expected.
I don't suppose you have an old router that could be recommissioned for the day of re-flashing? I have an old 150 Mbps 802.11n Linksys that serves that purpose; it's pretty much worthless on the secondary market but my laptops all remember its credentials so it got recomissioned last year when my RT66U got misplaced while moving.
