RMA Staff accounts are simply normal customer accounts tagged as "RMA-Admin"
This is because Shopify doesn't actually give us a way to see if the logged in user is a Staff member or customer. So we are forced to use customer accounts with tags to determine what that account has access to.
So to keep everything integrated your Staff members that you want to give management access to for RMA records will have a normal customer account and you tag them as an admin.
This does provide one bonus feature. Say you have a member you want to let manage RMA Records but you don't want them to be able to issue refunds or have access to all the other admin sections. Then you create them a normal customer account tagged as RMA-Admin because they cannot access the actual shop admin panel they will not be able to issue refunds.
Anyone logged in to a normal account that is tagged with RMA-Admin will have access to the integrated RMA Admin Controls
Simply go to /apps/rma/admin on your site and you will see those controls.
Now as for keeping customers from requesting an RMA Record you can do that by simply not modifying the accounts.liquid If your customer has no idea how to access the request form then they can't request one.
Your staff/rma admins would have to create them an RMA Record on site. I will need to add this feature to the RMA Admin Controls. They would login, choose an order and create an RMA. They would then give the customer the link to their RMA which that customer can only access if they login to their account.
As for your question of where the RMA tab is now on the admin panel. It is gone because those are tabs to off site applications. RMA is integrated so you simply access your RMA controls directly on your site using the Admin url ( /apps/rma/admin )