Multiple should be fine, as long as they have a common direction (common headline). I think you will struggle a bit more in finding a student for that, but I might be wrong. One thing gsoc is strict on, is that it should be features, not cleanup behind you or bugfix for you 
No, GSOC doesn’t care afaik. Just say elm-tooling is the umbrella organization in this case. One thing google expects, is that whatever get’s worked on is available somewhere when it’s done, it doesn’t even have to be git (but I would recommend git)