How to accomplish SQL Server 2012+2014 high availability
how to do this for a mixed environment of 2012 and 2014.


Doing HA in a mixed environment is a recipe for disaster because during disaster once you failover from lower (2012) to higher (2014) version, you cannot failback.

Since you are using Enterprise edition of SQL Server, you can leverage AlwaysON technology wherein you can configure writes occurring on Primary and reads on the secondary configured as readable secondary.

Thinking a bit more on what you mean by mixed, I presume that transactional replication would be much better from sql server 2014 as publisher and 2012 acting as subscriber. 

The performance would be dependent on how big is the database, if you are replicating only the tables that you need or the entire database, the latency between the two servers as well as the amount of transactions that are occurring on your publisher.

Note that if you go with T-Rep, doing schema changes will require a new snapshot. Also there will be a distribution database created as a part of setting up T-Rep.