Would it be possible for all letters in preview to revert to the master layer if you either click twice on the master button or use the shortcut (cmd+number) twice in a row? If you have any other layer than the master selected switching between masters doesn’t affect this one and it does make sense. It would be just faster if there was a shortcut to revert everything to the master layer.
Sounds like the job for a script:
currentTab = Glyphs.font.currentTab
currentTab.text = currentTab.text
It takes the text of the current tab as a
str and sets the text of the crest tab to that string. Since no master information is transported in a
str, it has to default to the current master.
It does work fine. I just realised that it can also be done by selecting all and choosing the equivalent master layer from the layers palette. You can delete the post.