It's done: after talking to both bidding contractors today, I told Betsy (former high bidder, now right in the range of the other two reasonable bids) that she has the job. The contractor I was leaning against called me to see if I'd made a decision before I'd had a chance to talk to the one I was leaning towards, so I had to just say 'no decision yet, but soon'. I'll call him tomorrow to say thanks, but no--and hope that I can just leave a voice mail, as I find it a very awkward call to make in person.
I've sent off an email to Betsy, cc:'ed to my architect, of the things I want changed from the plans. These include the necessary structural beam to replace the load-bearing wall, the cabinet changes, removing the (expensive and, I'm now convinced, unneeded) floor drain in the laundry room, and a possible addition of a warming drawer if the cabinet under the wall oven will hold it.
Betsy will start on the contract proper, and we'll meet next week and get that hammered out.