I'm working more in depth on my pronunciation of Quenya. When you have an -e- with a diaeresis -ë-. Example that I'm confused by, is it 'Manwë', rhyming with English 'we', or 'Manway' rhyming with English 'way'?
This is what I've gather but let me know if it's wrong: -e- in Quenya is always pronounced like 'eh', 'end' -- unless it has the diaeresis and is less than three syllables, then it is -e- as in 'me'.
For still there are so many things
that I have never seen:
in every wood in every spring
there is a different green.
There is no difference in pronunciation between e and ë. Tolkien chose to write some e's with the diaeresis ë as he saw that many English speakers didn't pronounce these e's correctly. So all e's are like the e in 'end'.
Yes Tolkien uses the diaeresis to (try to) make sure folks pronounce the -e at the end of certain words or names (it's not silent), or to illustrate that ea, for example, has two syllables (in Quenya ea is not a diphthong).
Tolkien himself did not always employ this mark however, nor is it found in Elvish writing.
I would say (concerning Manwe) it's not like English "we" and not exactly like English "way" either, but it's more like the latter even when Tolkien says it (in certain examples), but to my ear the e in end is a bit different than English "way".
That said, for writing some real world languages the same diacritic is used to designate a different sound, so it can get confusing for, say, German speakers (see umlaut) for instance.