I gather from the rules that a player can score a bonus card at any time.
In my first game, I had a situation where my opponent had two beans to plant into one of his fields at the end of his turn. He placed the first one, and immediately placed the second one on top of it.
In that brief second between the placement of the first and second card, I could declare my bonus card completed. But as soon as he placed the second card, my bonus card was no longer valid.
Can I claim a bonus card in the middle of my opponent's action in that way - or would I have to wait until he had finished his total action before I could claim?
you have to plant the beans one after another in order to allow bohnus cards to be played. So yes, you can score them in between without any problems. :)