I like DeMelo over Tanev just due to age, Toronto has had some bad luck with their d-man just falling off a cliff play wise due to style of play and mileage on them, I think Tanev falls into that category.
You're right, you can't solve every issue but being able to grab a top 4 dman without spending any assets is something Toronto needs to do. I want Toronto to get a real goalie, but I think with there needs are currently, that's going to be something they push to a later date. (Probably could get away running a cheap tandem and then making a trade @ the deadline for a goalie if needed).
I'm not even considering Marner off the team at this point, with what's came out so far, it seems pretty apparent the loser doesn't wanna go and theres nothing Toronto can do about it.