Why can't I create a Droplet from a snapshot?

You cannot create Droplets in certain datacenters due to limited capacity. If your snapshot is in a limited capacity datacenter, you can’t create a Droplet from that snapshot.

If you have a snapshot in a datacenter where Droplet creation is not available, transfer the snapshot to another datacenter. You can then create a new Droplet from that snapshot in the new datacenter.

You can see where Droplets are available on the Droplet availability page.

You cannot downsize a Droplet from a snapshot. Data is not always stored sequentially in memory, so reducing the size of a disk can result in data loss or corruption.
No, but you can use reserved IPs to assign the same address to new or redeployed Droplets.