[Quoth]

I use LibreOffice Writer. I've had no problem with any epub reader, or conversion to mobi or azw with any accented letter used in Europe-Scandinavia Iceland.
I just type them!
Some use the Linux Compose key.
The ONLY ones I can't type are the prime and double prime: ′ ″ so I use “Insert special character” or a charmap tool.

hyphen -
en –
em —
ellipsis …
áéíóú ÁÉÍÓÚ
àèìòù ÀÈÌÒÙ
üöëäï
ł Ł þ Þ ŧ Ŧ Ω ẃ Ẃ ý Ý ß § «»ç Ç “” ‘ ’ ñ Ñ ð Ð đ ª ŋ Ŋ ħ Ħ ĸ

You can copy & paste if you are on Windows or Mac. Try Spanish Keyboard layout on Windows. Above covers Scandinavian/Icelandic, Polish, French, German, Spanish, Irish etc. I may have missed a few.

Some greek letters will work. But not all, nor Cyrillic.
αβδ etc

Or are you looking for HTML entities?

I always edit using odt format with LO Writer, do an extra save As docx and use Calibre to make the epub.

I MIGHT occasionally edit CSS, but not needed if Styles in LO Writer used correctly. I never edit HTML now, not for over 15 years.