While I hope the Jays get Yu Darvish, there's a significant amount of risk with posting $50MM and then guaranteeing a contract close to $50MM to get him to Toronto. There's a much better risk/reward proposition with obtaining Felix Hernandez.. Hernandez is owed $58MM for the next three years. Assuming you have to pay $100MM to get Darvish for 3-4 years ($13-14MM/year), obtaining Hernandez would be more cost efficient.
The Mariners need help in the OF, could use more young starting pitching and need an impact bat. The Jays have all three. Travis Snider and Kyle Drabek both fulfill the young potential that the Mariners' are seeking. Snider is from the Seattle area as well. Drabek could benefit from pitching at a more spacious ballpark, just as another piece to the deal, Brett Cecil could. Cecil could win 12-15 games again if he was not pitching in the more difficult AL East. He also had two excellent starts last year against the Rangers so that bodes well for how he would pitch in the AL West. Finally adding Adam Lind to this deal would seal the deal. The Mariners need a 1B with pop and have been pursuing Prince Fielder. Lind gives you the potential for 3 WAR and 30 HR's at a reasonable contract for the next 4-5 seasons.
What would the Jays do at 1B? Well they'd have a few options. The most obvious is to sign Prince Fielder with the savings they gain by going after Hernandez instead of Darvish. I would not go more than 3 years with Fielder, but I would try and offer him 3 yr/$80MM. This way, he can still be a free agent at age 30 and when more teams might be able to give him a better offer and he could work on inflating his stats in a hitter friendly ballpark like the Rogers Centre. The other more intriguing option is to package more of the Jays depth and obtain Joey Votto. The Reds are in no rush to dealing him, but could use young pitching and probably need to unload some salary. They also have Yonder Alonso. You could do a straight up Alonso for Henderson Alvarez, but I'd rather see the Jays try and do a Deck McGuire, Colby Rasmus, and Adeiny Hechavarria deal for Joey Votto. The Reds would add two quality 2-3 starters and everyday position players in Thames and Hechavarria.
While you deplete the farm system of McGuire, Drabek, Snider, and Hechavarria you get Votto and Hernandez.. The Jays have enough depth on the pitching side to overcome the loss of McGuire and Drabek. Snider is probably going to blossom somewhere else and LF is now covered with the Thames/Francisco platoon. Hechavarria does deplete the middle IF depth, but it's case of selling high on a guy who may be an all-field/can't hit guy.
You could find a cheaper 1 yr option in CF such as Coco Crisp who could play CF and bat leadoff with some speed. Him and Rajai could be adequate in CF while we wait for Gose or Jake Marisnick to come in next year.