I agree whole heartedly - the dust is far supperior for treating nesting materials. Whether the garage is warmed or not.
As I said above, I have a bias against dust due to my climate/conditions. WHen I had to treat my hen house, I had to strip everything out, spray the whole thing, wait...
I don't like the dust - but I am in an always wet environment, which is why I use the spray. Also allows me to spray overhead corners when I treat a hen house or a goat house.
For your needs, @DobieLover 's advice is better than mine. Use the dust.
Yes, its ok to spray the nest.
Yes, you should keep the cat out of the garage. and if your are really worried, though this is overkill, change your clothes in the garage after spraying, throw them in the laundry, then go wash your hands (ok, washing your hands isn't overkill) up to your...