1) Even after 3) there still isn't a usable database because "mydatabase" on S2 (which had become the primary) could've had transactions on it that S1 doesn't know about yet?

Firstly, speakers invest a lot of their own time, mostly unpaid to give back to the community, and anyone who fails to appreciate this should stop attending sessions.

Start with Perry's Stairway to AlwaysOn, will answer all your questions: http://www.sqlservercentral.com/stairway/112556/

Try the following. I set @myvalue to 6 as a test for a normal operation. Comment it out and you'll get a "-3" returned.
Build your other logic around/in it. 

Since the Quorum and FSW is a Windows Cluster Component, your best bet is PowerShell:

get-clusterresource -cluster sqlcluster | where-object {$_.ResourceType -like “File Share Witness”} | get-clusterparameter

To...

If I understand your question correct, you should use ALTER AVAILABILITY GROUP:

https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/change-the-availability-mode-of-an-availability-replica-sql-server

From your post, we can understand that:
1. Your Infrastructure Team wants a Failover Cluster with a FSW Quorum instead of shared Disk, 
2. SQL DB Mirroring uses a Witness...

Consider using the ReadOnlyRouting and the ApplicationIntent=ReadOnly switch in your connection string for the read-only readers.