SQL Server 2012 database 3 mirroring questions
You can use SQL Server Express for the witness.

You only need a single license so long as you never use the mirror server for anything other than a mirror, and only failover at most once in 90 days.

The names of the servers, and the names of the SQL instances are irrelevant.

Access can somewhat automatically use the failover server as long as the connection string for the linked tables uses the SQL Native Client and the correct syntax:

Failover Partner=myMirrorServerAddress;


See http://www.connectionstrings.com for more info.