Priorities: first, dry, then dark, then not too warm.

Freezing shouldn't be a big problem.

I've stored many leftover batches in an unheated room off my garage without problems. The germination rate will decline some just because of time, but, it should still be useable next year if stored well.

Agree, just keep it dry and cool.

The garage will be fine for winter as long as you keep the seed dry. Since you want to use it next fall though, I might be concerned about how warm it gets in summer. Hot and humid summer air is no good for seed. You'll definitely want to seal it up good (not just fold over the top!)