Being paranoid with your PBNs is a good thing. First question is a non-issue, but to satisfy your paranoia -- users would still have the remaining time left in their subscription period, during which they'd have the opportunity to export their sites.
Second question is excellent. I can't comment on other PBN hosting providers, but we follow best practices for minimizing footprints on the server level -- of course providing unique IPs for every blog in a network from various providers, keeping updated with the latest security features, monitoring and blocking/cloaking bot access, etc. -- and we try to guide our users to do the same on the blog level.
If you have any other questions or concerns, please feel free to PM me.