I'm not suggesting Mario will provide anything more than Stiles could but I am sure they have enough respect for each other to work well. Plus Stiles has a lot more on his plate this year, so if Mario can help cover off on the scrum while the HC is doing other stuff (coming up with a match strategy etc), then that's great.
Check could just be a great coaching mentor for Stiles, if nothing else. He has achieved some great things and Nick is new to the HC role, particularly at that level. Check may not be on the field at all, but if he can be there just to support Nick, then that's great.
Byrne has already started his work and, I agree, his role is vital