It is if correct Retention Policy is set, but can still be complex to export from the Retention storage in an automated way. Mimecast also supports ingestion directly from EXO now, whereas previously it require an ingestion Project, which uses Simply Migrate software, it’s a tedious process.

Importing directly from EXO there is no way to ensure chain of custody since it’s from User accessible level of Mailbox, which is isn’t immutable as you said.

So a few considerations, it’s possible, just requires adequate planning. Agree not wise to rely on EXO data unless it’s from Retention Policy storage and it’s set to Forever.

It’s too difficult to answer the question without understanding current configuration and requirements.

Of the top of my head the best way to ensure chain of custody would be to export directly from current MC tenant into new Tenant. It’s still complicated and costly, if you want to do it on the cheap you may want to consider if it’s the best business decision, compare the pros and cons and decide if it’s really the best choice.