No, no need to screw it since weight will suffice to keep it stable. That is assuming that the posts will be supporting a platform of some type. If you are doing this where a permit is required you might not be able to use these since the post may need to be anchored to a concrete footing.
Hole is 2" deep so post sits 6" off ground. I think that home depot uses different brands or manufactures for these blocks depending on whats availble locally so I would recommend calling or visiting your local store to be sure its the same. Might not be same depth for all brands.
I live in connecticut and its 6.30 each here, but price depends on location. Not even available in all locations. Try the app, once you select your local store it will give you a exact price.
No, waste of time. You probably will not have a problem with frost heave, but if you are doing this with a permit you may have to dig a footing to code in your area.
No. Footings in any locale are required to be beneath the local frost line to meet standard codes.
NO, provides little horizonal support.
The price vairies based on location. The homedepot app will tell you the price for your area when you select your local store. Here on east coast in CT it's $6.30 each.