No, they will be fine, temp wise. The snow will actually insulate them. If any are blooming, I would bring them inside because they might snap off in the heavy snow : ( Keriann~

Plastic really isn't the best thermal cover -- it doesn't really insulate. If you have to use plastic, be sure to use some kind of framework to keep it up off the plants and create a dead air space for insulation. Plastic directly on plants during a cold snap actually cause more damage, IMO, as frost and ice form right on the plastic and burn the plants through it. Nothing on your list should be harmed by snow or temperatures that fall into the mid 20's. Snow actually insulates. I wouldn't worry too much, should be OK -- check your local NWS site for a specific forecast of the lows you might get in the next few nights and see what kind of measures you might need to take.

It should be fine. Search "dormant seeding."

The only problem you can usually get from snow is when it melts, the bulbs get heaved up and dry out since the freeze thaw cycle caused bulbs that are planted too shallow to come upwards, but onions and garlic should be hardy enough to take anything in zone 8b so I don't think there's any issues with that. If they are heaved up, just press them back into the soil and they will be fine. Snow actually serves as an insulator for garden plants. Here is a link that might be useful: Here's some pictures of how I protect my garlic