Oi! Hands off...
a) If I have 2 farmers in my guildhall and now I play a third farmer in my action area, will this trigger the "1 farmer" ability and provide me with one VP?
Yes. All the '1 xxxx' abilities actually should be read as '1 OR MORE xxxxx'
b) If I have 3 assassins in my guildhall and now I play a fourth assassin in my action area, will this trigger the "2 assassins" ability and let me discard one card each from two different chapters of one other player?
Yes. As above.
2. If my played card ability affects the cards of "one other player" that means it has to be one and the same other player. Right?
If I trigger the "2 assassins" ability I can only discard one card each of two different chapters of one and the same player. I cannot discard one card from player A and one card from player B. Right?