I really have no idea what you are trying to do here. Overriding capabilities that a user has in the 'user' context, will not affect what roles they are assigned in the 'course' context.
You are mixing up both capabilities and roles, as well as assigning something in one context and expecting it to affect a completely different context.
If you are trying to grant a user access to the MRBS block, then you are going about it in the wrong way. The MRBS block works only at the SYSTEM level - it does not matter what roles you assign at the user, course or any other level, if the user does not have the right capability to access the block at the SYSTEM level then they will not be able to access the block.
To help you out, there are 3 custom roles created at the system level, with the right mix of capabilities for the common situations that are needed - an MRBS viewer, MRBS editor and MRBS administrator.
To use these roles, click on 'Site administration > Users > Permissions > Assign System Roles' then assign the appropriate role to the appropriate users on this screen.