When working on filling glyph slots, diacritics are displayed at the bottom of a language category, but not their variants. This is inconvenient when e.g. having .case variants and starting work on the upper case letters.
Language entries are glyph name lists. It is impossible to predict which variants may be needed in a file. (You can make your own list filters and language entries though, see the tutorials.)
For managing a class of glyphs, like marks, use a category, in this case I would use Categories > Mark > Combining.
Not sure what you mean; In e.g. Languages > Latin > Western Europe, Glyphs could just display all glyphs with names starting with “acutecomb”. That would include “acutecomb.case” etc. For seomthing more elegant, scan all non-mark glyphs and only display the combining marks contained in them.
Two problems: (A) If you want corresponding .case alternates for all comb accents, this would be an inefficient (you would have to go through all Language groups) and potentially incomplete (would only catch the accents in those groups) method. (B) By far not every design needs separate case variants for every accent.
But there is a better way already. You can simply go to Categories > Mark > Combining, duplicate the comb marks, and rename .001 to .case, then update the features and/or recreate the cap compounds.
Do you mean that the .case accents should be in the missing glyph list or just show up when you select what category?
Show up when you select the category.
E.g. a font contains the base Latin glyphs plus everything in the Western Europe set plus acutecomb.case and gravecomb.case. The latter two should show up in the glyph set view when I select a language group that contains acutes and graves. No modification to the “missing glyphs” list or anything, they just should show up alongside the base mark glyph if they exist.