1. yes, but it depends on your requirements
2. correct
3. "
Configuration changes cannot be made on the secondary Mobility Master. In a scenario where the primary Mobility Master is down and configuration changes need to be made on the secondary Mobility Master the user must change the sync state of the secondary Mobility Master to primary.
To preserve these configuration changes, a Layer-3 synchronization between the new primary Mobility Master and the old primary Mobility Master should take place. For the synchronization to take place the sync state of the old primary Mobility Master should be changed from primary to secondary state."
4. I would not deploy l3 redundant MMs. I would deploy layer 2 redundant MMs in a hardened site so that you do not have to configure anything manually or switch state manually. I believe in my heart l3 MMs is not for most people, only for the few that demand it, but enabling configuration manually (with l3 redundancy) is a non-starter for most.
EDIT: My information is from the page here: https://www.arubanetworks.com/techdocs/ArubaOS_83_Web_Help/content/arubaframestyles/vrrp/config_stand_mm_l3_redund.htm
Having Layer 3 redundant infrastructure where you have to maintain state could become a "split brain" issue that you don't want to have to manage.