2.) Exchange 2010 is supported https://docs.microsoft.com/en-us/exchange/hybrid-deployment-prerequisites. As stated in the article: Hybrid deployments require the latest cumulative update or update rollup available for the version of Exchange you have installed in your on-premises organization. If you can't install the latest cumulative update or update rollup, the immediately previous release is also supported. Older cumulative updates or update rollups aren't supported.
Question 1. Using Remote-Mailbox, can this be done after Exchange Hybrid has been decommissioned? What server roles must be left on an Exchange management server in order to do this as remote-mailbox is only available via Exchange 2010 PowerShell
Question 2. Do I simply install Exchange 2016 on the hybrid server to get access to the key?