Regarding Vaishnava aparadha, offending Krishna's devotees: Would someone who doesn't know what this means (or isn't yet a devotee of Krishna) still be punished for this?

Vaishnav Aparadha is a very dangerous thing. Apa means against and radha means devotion; so it is an act against devotion or an act that stops devotion.

Generally, Vaishnava aparadha implies some type of deliberate ill intent— the desire to hurt someone, or find fault with them in order to make oneself feel better, or look better in the eyes of others, to get even, or retaliate. In each of these cases there's an element of envy and malice. Whether one knows about aparadha or not, if envy and malice are there then there will be a reaction. And yes, it also applies to nondevotees. In once sense, it applies mainly to nondevotees because real devotees don't commit Vaishnava aparadha.

If one offends another Vaishnava, and is aware one has done so, the best thing is to go and beg for forgiveness, if this is not possible then sincere chanting of Hare Krishna, and begging the Lord for forgiveness is in order.

I hope this is helpful.
Sincerely
Laxmimoni dasi