opinion not withstanding

I suppose I meant that umlaut is a word that describes both an historical phonological process and a diacritical mark used to indicate the modification of a sound (grapheme). I've never heard diaeresis used in this way for the former, but even anglophone linguists use the term umalut in this way to describe how the plural for foot turned out feet and not foots. In fact, umlaut in English is not associated with anything diacritical in the orthography. Adding a diaeresis over the second o in coöperation merely reveals yet another inadequacy of English orthography, not some some bit of phonological history of English. Same with Brontë and résumé, both of which could easily be Bronté and resumë.

As for ligatures, while æ (or ash) and œ have stuck it out, sort of, the long s short s and the ct ligatures have not. There are still fi, fl, and ffl lurking around in some fonts. I believe ligatures are a holdover from MS days, and currently exist in English for aesthetics.

The term cedilla is from the Spanish word for 'little zed' which is the origin of the little mark under the c.

Ceci n'est pas un seing.