If I have no cards in hand, do I have to kneel a character of Str 3 or higher, or do I get off scot free?

My understanding is that you don't ever need to kneel a character. And if your hand is empty, well, then you don't pay the penalty, either.

This is correct for the way Alannys is worded. It's better to think of Alannys as a discard effect, not a kneel effect. Effectively, kneeling the character is an optional "block" to discarding a card from hand. If you don't block, but have nothing to discard, oh well.