Hi!
Sorry for delay in responding...
1. blockstore.default_namespace_prefix lets you set the default prefix. (We added it for this very purpose in the original playground, iirc).
2. Any given installation of lakeFS uses a single blockstore. (Your probably wouldn't want to live in AWS and access storage in Azure...)
3. We have no functionality to force all repositories to use this prefix.
4. You can of course use an appropriately configured role to prevent lakeFS from accessing anything outside a given prefix.
Hope this answers your question!