Well, it was paradox decision to do war goal system like that, regardless of how many states join, and size of war. Why not allow multiple wargoals and addition in process is slightly beyond me, but probably the system of "diplomatic plays" didn't work then.
There "should" be, but there are none, paradox couldn't even copy volunteer mechanic from HOI4. Besides, the lovely building based warfare system can't support providing arms and advisers.
Sure, but we will probably get that in Great war DLC.