Hosting Multiple MongoDB Databases on the same MongoDB Instance/Cluster
Q1. what about the question of having multiple production databases on the same instance? 

A:  It is very normal to have multiple databases on same instance.

Q2. Creating a User/Role that is limited in access to someDatabase
the above considered a normal and standard way of providing a logical &amp; security barrier between tenants?

A: Database level authorization, works very effectively for limiting the user access rights on databases, it is safe to use.

Q3. If I needed to migrate one of these logical DBs to its own MongoDB instance, would that be possible?

A. You cannot have two databases with same name on same instance or cluster. 

(If more details required, do ask)