Its value calculate using the formula N/2+1. Where N is the number of master node. To avoid from split brain problem we should have minimum 3 master node. Then discovery.zen.minimum_master_nodes value will be 2 (3/2+1=2).
Yes, you need to take the backup of your data. Because when you upgrade your ELK all data will be lost. I am not sure whether you can restore the elasticserach1.5.2 snapshots in latest elasticsearch or not. The latest elasticsearch version is 6.5.0.
Yes, when you have multiple data node in your cluster the elasticsearch itself take care of data distributing across all data nodes.
Kindly let me know if i can help you more.