If you uploaded files using GatewayMT, you may get checksum from ETags,
You may see implemented methods there:
Yes, it may be possible, but availability and other questions you need to discuss with sales.
Unlikely. In a Global Storj Network each piece of every segment of the file is stored on independent nodes, not in a single place, so the durability is much higher than any centralized storage solution.
Yes, it’s possible and available, however, you need to contact sales anyway.