Well the team has lost 3 of 4 so there's a few players who we can point fingers at and say they were guilty of many of those issues you stated up above.
The part about his size/weight essentially is "unfixable" in the WHL. At 19 I don't expect him to put on a lot more weight or height this year regardless if he plays in junior or the NHL.
I honestly think he's learning more by experiencing the NHL pace and intensity and learning NHL caliber systems in successive games (and taking part in hard NHL practices) than he would learn by playing at the WHL level against lesser competition and with WHL level teammates.
Who better to learn the game at the NHL level than with the fastest, most dynamic NHL player around.
If Yamamoto can keep up with McDavid and play well with him... that's as good as it gets in terms of NHL development imo.
Does that help the team? Can't see how it hurts them if he is generating shots/scoring chances and points.
I do agree that he has to be doing well though... not just going through the motions, but actually producing dangerous offensive chances and getting points on the board in order for him to stay with the team for the full season. So far I see that beginning to happen with the way he plays and I think that can and will continue.
He also made a smart defensive play last game coming back and helping break up a shorthanded chance against that I'm sure caught the eyes of coaches. Those plays will earn him a job almost as fast as scoring goals will. His all around awareness at both ends has to be solid and so far he's moving in the right direction.