The SDF files can remain basically the same, but the plugins need to be completely rewritten. It’s currently not trivial to port plugins and that’s one of the things we’re working to improve. Meanwhile, here are some relevant resources:
Yes, like Gazebo-classic, Ignition uses a client-server architecture, so you can choose to run simulation headless (i.e. ign gazebo -s for server only and ign gazebo -g for GUI only). That’s currently being used to run SubT in the cloud, for example.
Here’s the Ignition roadmap. The Citadel section is being updated with more bullets soon.
You should already be able to build ros_ign from source on OSX for dashing. If that’s not working, please open an issue on that repository with the specific problem you’re seeing (or even better, open a PR fixing it 
).