Really helpful - thanks. The problem (such as it is) is that during the purchase of the ticket, there is no indication that IB is recognizing the OW status: no seat offer, nothing about baggage. Then, when the tickets are generated as pdf files sent to your email after purchase, they explicitly say no checked bags.

This said, I went back into the IB web site, went to Manage Bookings, put in the reservation confirmation number, and was pleased to see that it could find the reservation and was showing us each with a checked bag. I was also able to select XL seats for both of us, and they seem to have "stuck," at least for now.