Dave - I had a site report to me that their teachers (mrbs editors) were not able to access the scheduling. I noticed that in web/functions.php you added a require capability for viewing. In this case, the site admin had not added that capability to the mrbs/editor role because it says read/write. I'm not sure if it would be better to check for either read or read/write or separate the functionality read and write and require both.

block/mrbs:editmrbs - Access MRBS (Read / Write)
block/mrbs:viewmrbs - Access MRBS (Read only)

Perhaps it would be better to simply change the editmrbs to say that it is for Write rather than Read / Write to avoid confusion. That would avoid the need to recode things.

Peace - Anthony