Can I use SQL 2008 R2 as a Witness Server for 2 instances of SQL 2012 Servers being mirrored?
No witness server has to be SQL 2012, you can use SQL Express 2012 though
All server participating in database mirroring should be running same version of SQL server.
Taken from Microsoft SQL Server BOL
Database Mirroring will be deprecated in future release of SQL Server if possible use AlwasyOn Availability Group