Thank you for your advice! Feels solid enough - definitely not concerned about working in/around it now.
In order to make sure this is taken care of before we leave town on Sat., I took half a day today. Put down the sill seal, with a 1/2" strip of butyl squished along the outer edge.
new pressured treated 2x10 sill plate, 8' section starting from the East end. 4x4" post in place, and dimensional 2x4" post for the door frame. Wall studs still to be addressed.
Outer panels in place for this corner. Sealed the sill at the corner also. Have to get new siding for the bottom 1' of the wall - I'm going to use concrete backer board for at least the lowest run (6") off the sill
These garage doors have been unusable for at least 10 years, wedged shut. Now I can open & close them (after trimming the righthand one), I need to address the latches to properly secure them, and replace the pane of glass I broke.