Dear @Hubert Dudek, thanks for your response. We have performed your suggested points one and two. With regards to mounting storage using the app registration we've followed prober instructions.
Interestingly, today I created a Databricks resource without VNET injection, and allowed all connections from Azure services to the storage account. Via this new workspace I am able to mount and read data from the storage account. Reading from the VNET injected Databricks workspace remains unsuccesful.
We are open to more suggestions.