Luggage on the current Skytrain cars is doable IMO. I just wouldn't do it during a weekday rush hour. Navigating the escalator might be tricky but I think each station should have an elevator.
The walk from your hotel to the Stadium Skytrain station is easy.
Mind you, you'd only be saving about $10 I think ($15 cab ride - $2.50x2 tickets).

BTW, Granville Island really isn't an island per se. It's just a piece of land under the Granville Street Bridge that juts into False Creek. If you enter along the main entrance along the sidewalk (this is the only way in/out for cars), you have to go over a small bridge by a marina. Just before the bridge is a seawall walkway along the marina. The path should take you to Go Fish! in <5mins. Warning though: They basically cook out of a portable/shack so there is only outdoor seating (go only if the weather is nice). The open at 11am or noon (closed Mon) and the lineups/wait for the food can be killer (30mins?) because they make to order. They have halibut & chips as well as cod and salmon. Have to recommend their soup and fish tacones also.