1. Recommendation change to 15.1 was done, then Juniper started seeing issues with 15.1. There is a KB out specific to EX2200 to NOT run 15.1 on this platforn. I [personally] am recommending not to run 15.1 on any legacy EX, at this time.
2. I know it works because I've done it. 12.3 to be upgraded directly to 15.1 without issues. The "rules" are based upon what is tested, not neccesarily on what may work.
3. No comment