I see Fleet supports using the OCI format for Helm charts. Can Fleet reference a private Container Registry (Azure Container Registry) for the OCI charts? And will Fleet support signed OCI container with Notary v1 and/or v2?

Fleet does support user authentication for OCI based registries - as described in the "Private Helm Repo" box under https://fleet.rancher.io/gitrepo-structure/ . Verification of signatures is not implemented and not planned at the moment to my knowledge.