I'm not sure if Rielly is ideal for captain but this does make a lot of sense, after all, if we don't get Stamkos there are no other reasonable candidates on the horizon and I don't think they want to go for years without a captain. It does seem entirely that Morgan will start the season with an A, maybe Kadri too.
I don't know, why don't you ask them?
If it was any other player drafted since Crosby I'd agree with you but McDavid is different and extra special. I doubt very much his teammates would have any problem with the inevitable happening sooner as opposed to later.
PS I just looked it up, Crosby was made captain at age 19, worked out OK for him, it'll work out just fine for McDavid as well.