Not irregular, some nouns in -η form plural with -ες and some others with -εις. there is really no way to know beforehand, you just have to learn them by heart.
That's true, but there are also exceptions the other way around: βρύση-βρύσες. But most of the times what you say is true. ;)
Greek word απαντά can be translated in English as : 1. Answer(s) 2. Reply/replies. Therefore both words should be accepted.