Georg Seifert

Glyphs doesn't need to do anything to support any curve. Just go to Pablo's website and let is calculate the instance values. And there is a script that can do the setup right inside glyphs. 

I wonder if the "national preference" for certain accents is caused by the preference for certain typefaces that happen to have certain accents and users get used to the style of that typeface?

I put it on my webserver: https://files.glyphsapp.com/FontValidator0.1-2.zip

There is a BDF plugin for Glyphs.app, too: https://github.com/schriftgestalt/BDFFileFormat

If you need two or three masters on the weight axis depends on the design of the font. So you can try to us only two, interpolate the instances and see if you like the result. If not, you add add the Regular in the middle. If you need corrections only on some glyphs, you can add add extra 'masters' per glyph (in Glyphs.app with a brace layer and with an extra UFO in designspace files, I believe). That way you save a lot work.