In terms of Mantha, I can understand the hesitation as he doesn't have a well rounded game and thinks primarily offence. But the upside is there. His skating is not much of an issue, really, although he definitely lurks waiting for his chances (yes, some would call it floating).
The reality though, is he play a lot like an 18 year old Dany Heatley. I don't think he has quite the puck skills as a young Heatley, but his skating is probably better and 50 goals in 67 games is serious production for a draft eligible (as good as anyone else in this class).
Would I take him at 17? Probably not in this draft with some of the talent that will be there, and partially because I also recognize the risk that if the skill is just short of where it needs to be at the next level, he has some serious bust potential. A lot of digging has to be done by your scouts.
But again, Redline is not a fan of but begrudgingly admits he might still score 30 at the NHL. But it will be a Michael Ryder like 30, where as if he isn't scoring for you, he isn't do anything for you.
As others have said, I would be personally pretty surprised if the Sens scouts were big on him. Not really the way the Sens are looking to build this team.