Can auto failover cause split-brain? (SQL AlwaysOn)
This following from Microsoft seems to indicate that only a person with SA, doing something stupid can cause it. 


Yes, or doing completely unsupported things like using DSFR on a fileshare witness. If you're staying in supported land and aren't going around forcing stuff up, the worst case is the entire cluster will be offline until an admin can intervene.


  There are a lot of variables, and I wonder if something like changing the Dynamic quorum (is on by default) could create a scenario where a split-brain scenario exist.


I wouldn't turn this off, it's actually your friend. This stops the need from having to change the node weight per node every time something happens in the cluster. BTW, you can't turn dynamic witness off.

There are extremely rare times that I would even consider turning dynamic quorum off and in those times it's because we've looked at the way we want the cluster to work and we specifically want it to fail if certain things happen. It's not something that you'd want to do without great testing and consideration.


  Can auto failover be configured so a split-brain scenario is possible? 


Absolutely not.