1. Are you still using Inkscape 0.91, as it says in your signature? If so, please update before writing any extensions, or you may run against some walls later. Inkscape 0.92.3 will be released within the next couple of days.
2. The correct folder to put user-generated extensions into can be looked up in Inkscape: Edit > Preferences > System: User extensions.
3. Move your extensions out of the other folder before you uninstall Inkscape for installing the new Inkscape version, else they will probably be deleted.
4. Put them into a separate folder somewhere on your hard disk where you want to have your git repositories.
5. Use symlinks that you put into the folder mentioned in 2) for the .py and the .inx file.
6. Sorry, can't help with Ruby -> Python.
7. You could also use Ruby for your Inkscape extensions. This means, though, that you cannot use the functionality provided by inkex.py, but must create your own equivalent to it.