> ...Those would be one-to-one mappings and many-to-one mappings, respectively. Would this general solution involve other kinds of GSUB mappings? ...
> Even sticking to just many-to-one and one-to-one mappings, ...
Having too exclusively a western-european language background might be a blind-side... I haven't followed the conversations too closely, but the 3rd scenario you try to ignore has a very common name: ligature. Ie. Special glyphs for combinations of "fi", "ff", for example. To recap, common usages:
One-to-one: alternates
Many-to-one: accent marks, e.g. umlauts
One-glyph-to-many-unicode-characters: ligatures, e.g. "ff", "fi".