Question on Pronunciation of the Long E Sound

I know there is no one "correct" pronunciation of Ancient Greek because of dialects and reconstruction, but I understand there is a pronunciation that understands "η" to make the same sound as "ε" but held for twice as long ("a long e sound"). I also know that (in the classical period?) ει wasn't actually pronounced as a dipthong with ε + ι, but instead made the same long e sound. I've also heard (at least in Attic? not sure if it also happens in Ionic, etc. with ηι) that the iota subscript on ῃ started to fade in pronunciation, meaning ῃ was pronounced as η, making the same long e sound. Do all three of these really make the same sound (in this particular period/theory on ancient pronunciation)?