This seems to have significant overlap with https://www.mediawiki.org/wiki/Requests_for_comment/Improving_extension_mana... .
Personally I think we should standardize around one tool and Composer seems the best positioned for that. So instead of maintaining N competing solutions with different strengths and weaknesses and leave the user to ponder which one to use, I think it would make more sense to focus on the holes in the Composer workflow and write small tools to plug them (e.g. a web interface to use Composer on servers without a shell access, or a way to disable/re-enable extensions without fully uninstalling them).
In that spirit, maybe writing a composer.json (or composer.lock) generator could be a good way of making the extension + git branch selection user friendly? Once you know which version of which extension to install, Composer should be able to do the job. The generator could even be a web service with a nice GUI, like the short url builder (which rocks by the way, thanks for maintaining it!)