A: vSphere HA and Storage DRS are not supported in a mixed-version cluster, and for good reason. Here's a quote from the vSphere 5 Documentation Center that explains what might happen:

vSphere HA might respond to a host failure by restarting a virtual machine on a host with an ESXi version different from the one on which the virtual machine was running before the failure. A problem can occur if, at the time of failure, the virtual machine was involved in a Storage vMotion action on an ESXi 5.0 host, and vSphere HA restarts the virtual machine on a host with a version prior to ESXi 5.0. While the virtual machine might power on, any subsequent attempts at snapshot operations could corrupt the vDisk state and leave the virtual machine unusable. (Best Practices for vSphere HA Clusters)

As a general rule, it's a good idea to ensure that all hosts in a vSphere cluster are upgraded before enabling any new features and functionality in the upgraded version.