Ya... it's not just raw physical (getting bigger and bulkier) development I'm talking about... it's that experience and maturation level where they get more aware and "street smart" on the ice.
I don't know if he is going to gain any additional mental awareness playing in Juniors. The guy already seems to think the game very, very well, and I think the biggest reason to send him down to the Juniors would be if he has any physical deficiencies (i.e. shot power, physical skills like stickhandling, shooting, passing, or physical strength) since being able to work on that is pretty independent on where you are playing.
I don't think there will be much of an advantage from a mental street smart perspective while still in juniors.
The kid's NHL ready. Hes only going to develop to his full potential against 'NHL caliber players'. This next season, as an 18 year old, will be crucial to his long term development.
I hope he plays in the NHL because that is what will be best for him this season, and long term... I will wait 'till his 9 game tryout stint is over to make my own personal opinion final but as of now I think it wont be beneficial to him, in the least, to go back to junior.
The only plus I can see about RNH returning to the WHL is guaranteed action in the WJC's. He still can do that if he's playing in the NHL, however.
His development would actually stall if sent back to junior and no matter what anyone says, there will be 'dejection' and the 'humiliation factor' of a first overall pick not making the cut.