Շատ գիտես՝ քիչ խոսիր

Yes, but often in a slightly more casual or even ironic way. However, the core message remains respected.

No, it would be considered disrespectful to give this kind of advice to a teacher.

'Խոսիր' is the formal/literary imperative, while 'խոսա' is the colloquial/spoken version.

Not at all! It means your words should have value. Quality over quantity.

That would be 'Չգիտես՝ մի՛ խոսիր' (You don't know, don't speak), which is much more blunt.

The dash (—) in Armenian punctuation often replaces a missing verb or conjunction, in this case, the word 'if' (եթե).

While not directly from the Bible, it aligns perfectly with many Christian values found in the Armenian Church.

It's a bit too informal/proverbial for a standard email, unless you are writing to a close colleague.

There isn't a direct 'proverb' opposite, but 'Դատարկ տակառը շատ է ձայն տալիս' (An empty barrel makes the most noise) is the negative counterpart.

Լռությունը ոսկի է (Lrrutyune voski e).