I wouldn't worry about the shelving material not being able to withstand the temperature changes, it is the moisture that is a problem, and being in the garage, and being sealed would give you no problems. The problem, as I see it, is the size of the plywood being 48"x96" and no room for error with a saw kerf. The answer is simple, use indoboard (industrial particle board) which is 97"x49". The only problem....I don't know if the big box stores carry it. But you should be able to find it, it is used by many cabinet makers. One problem, it is really heavy. I would think wherever you buy it they could cross-cut it in half making the 48-1/2" pieces easier to handle. If you have trouble finding it in your area, just contact a cabinet shop and they could tell you where they get theirs.
Best of Luck,