I get your philosophy... but wouldn't you trade Tavares for McDavid and Puljujarvi? I'm 100% sure that out of 30 managers, the number of those who would do it is 30.
And Matthews+Marner is not that far different. But then again, I get your philosophy, and the carefulness with getting the essentially unknown. As you say it could turn out wrong and it could turn out well, and it would probably depend on the mindset of Snow, what he would believe. As for my honest opinion, I do believe that Tavares isn't getting nowhere, he's the icon of the NYI and a trade to Toronto doesn't really make sense now anyway, for neither side, so it's basically just a debate in terms of, what, in theory, would it take. Of course the Islanders would want a lot coming back, which, due to the Leafs' own future plan, would probably not be happening.
With regard to your proposal, I disagree with you because I think that, for example, Matthews and Rielly is something Snow would simply have to think about, obviously I'm a little different because I value potential a lot, and Matthews, in my opinion, is very likely to be a player who's two-way ability and leadership is Toews-like, who's skills are Tavares or even Malkin-like, and who's maybe better skater than all those three guys. With prospects, there will almost never be a 100% certainity, if you insinuate that. But given Matthews' attitude, his work-ethic, his results as a player so far, I think the likelihood of fulfilling his potential is 80/20 in percents. I personally believe Snow would agree with Rielly+Matthews for Tavares, but I think Toronto would never propose that in the first plae.
And what deal would make you feel comfortable?