Hi Madan,
1. GRAC_OTHER_APPROVERS, I am not sure as I didn't use it much but I assume we can use it during RETURN notification event whereas we used GRAC_CURRENT_APPROVERS for RETURN event as well. May be we will check what other consultants suggest.
2. For points 2 and 3, you can check the below discussion which has detailed explanation.
ARQ: MSMP Workflow version Activation and Existing Requests' Fate???
Regards,
Madhu.