Can we switch manually from one QUEUE to Another QUEUE when resources are not available ? Example - i am running a query in DEV1, taking time so would like to switch it to DEV2 where resources are free.

[#1]

Yes its possible to switch between different queue and make use of the resources. But the user which you are using to access hive should have access to other queue which you are trying to switch, which will be decided by the person who has access to ambari- admin console. To try hive --hiveconf tez.queue.name=DEV2.

Choosing Tez or mapred depends on your need. If the user doesnt have access then the query will fail.