Is it possible to create an access request form that allows access request ONLY for Business Roles, not the other entitlements?
If not, is there a way to disable "Add Entitlements" button for certain users (Line Managers) while allowing them to add/remove business roles for users "who are their subordinates (only)"
The Add entitlment button is defined in the Request->Configuration->Buttons. Edit the availability to limit the display to Line Managers.
There is a view "Direct Subordinates". You can use this one for the user display. Unfortunate the "All users" and "All active users" views display all the users, no matter of what permissions the requestor has in IDM.