HC-110 is as good as any and better than most for developing old film. 6 min in B with constant sloshing in a tray. Occasionally lift and drain the plate and put it back in the tray the other way round - though I doubt developing evenness counts for much.
There is a chance the plates are ortho and you can develop them under a red safelight.
I don't leave film holders lying about with exposed film in them. I do find them lying about with unexposed film that I loaded in 1994. 5x7 glass plates probably means they were loaded pre WW-II. And you have to ask how many times someone pulled back the darkslide in broad daylight, wondering if the holders were loaded?
http://www.westfordcomp.com/ does a lot of developing of old exposed film - don't know if he has done glass plates, though, and the oldest on the website probably late 40's.