Hi ! Thank you for the nice words 
Yes, the typical case is you get an application, that can be very complex and composed of multiple containers, and then you have multiple devices running the same application. The multicontainer deployment can use all the resources available on the device, if needed.
The device can belong to a one application at a time, this is, among other things, to have a direct link between your source code and the device that is running it.
In other words, if you have a device - you always know what code it is running. We do not support having multiple applications on a single device, multiple services though are supported, via a multicontainer deployments.
Most of the things supported by the docker-compose file are supported. One notable exception, that we’re working on now, is supporting Dockerfile in build section - you can see forum thread here: Defining context in docker compose - Error: Cannot locate specified Dockerfile: Dockerfile
For the git submodules approach you can try doing it in runtime, as per this forum thread: Cloning git submodules when pushing to resin.io - I would recommend starting here actually and see how it goes ?