I'm posting this since it's not in the list of known bugs, but it's a low-priority bug.
Urchin and the "+" tokens from Adventures have the same timing. (This was a change some time ago, but I don't remember when.)
So: When you have Urchin in play and play an Attack that has the +1 Card token, you should be able to choose to trash the Urchin for a Mercenary before you draw the card. (Maybe you want to gain the Mercenary before you reshuffle.) At the moment you always draw before you get the choice.