Yeah, I cannot remember any instance either that Robin suspected foul play before.

Concerning Marian, Robin might mean yet another matter... he says it "cannot be undone"... even if Gisborne were sentenced as a traitor later, he still would have married, taken to his bed (and potentially fathered a child with) Marian...

And yes, I have now this picture in my mind with Richard coming back and being a total idiot... (this might just as well be a plot bunny *runs off to write*)