1. Do I have to upgrade the vCenter Server first to version 6 before upgrading the hosts to the new ESXi version?
Yes, you must upgrade the vCenter server first: VMware KB: Update sequence for vSphere 6.0 and its compatible VMware products
2. Can vcenter Server manage ESXi hosts with versions higher than the vCenter version is using?
No, it can manage only host at the same or lower level version.
1. Do I need to have the licenses for SRM for PROD and DR sites in both failback and reprotect scenarios?
It depends, but if you want protect VMs only from PROD site and then do failback, you can use the same license of the PROD site, see details here: https://www.vmware.com/files/pdf/products/SRM/vmware-site-recovery-manager-customer-faq.pdf
2. Is SRM license will be the same for both PROD and DR sites?
Depends if you're using uni-directional or bi-directional protection, see here again: https://www.vmware.com/files/pdf/products/SRM/vmware-site-recovery-manager-customer-faq.pdf
3. Do I need to create placement datastore in both replication using array and vsphere replication scenarios?
Yes for both, see: Select a Placeholder Datastore
4. If yes, do both sites need to have at least one placement datastore in failback and reprotect scenarios?
If you want use the reprotect, you need create placeholder datastore on protected site too.
5. What is the use of placement datastore? Do it have to be large?
Placeholder datastore is used to store placeholder virtual machine files, and since these files are just configuration files, your placeholder datastore don't need to be large and another recommendation is to not replicate the placeholder datastore. Additional details: About Placeholder Virtual Machines