I had considered thumb shifters but I really wanted to keep the handlebars as sparsely populated as possible...

The shifter clamp won't fit the stem, the stem is .833" diameter...ya know, funny schwinn sizes.

You are correct that the place i have it mounted in the pic is right in front of the brake cable stop, but It won't be an issue since the housing will just go over top of the shifter clamp band.

I'll definitely need a clamp-on cable stop if they're going on the top tube though, I just don't know what to look for...