Lots. The Fathers didn't die off in the 10th century.
Comparatively speaking, not lots. And in terms of Scripture, nothing at all.
Au contraire. II Esdras, in the Slavonic Bible, has not survived in Greek.
The Greek language is indispensable for a decent theological education
I would agree. But since it is not the Greek they are speaking in Greece, what do they do?
which is what our seminaries should offer our priests, Slavonic is not.
Perhaps not. The Russian theologians write in Russian now, and more becomes available in English and French.
It would be wonderful if priests of the Greek churches could acquaint themselves with the writings of Russian fathers in the original language (Fr. Sophrony of Essex, to pick one of the most recent examples), but surely from the pov of theological education they should master Syriac or Latin before embarking on Slavonic, Russian, or other languages.
I would prefer priests pay attention to their pastoring, for which translations suffice. That is, if they are good translations.