Thank you so much for your words!
To do the trick that you mention, every time the player touches a coin I check for collisions in the top of the coin stack, the position where that coin would be placed if picked. If it encounters a wall of any kind, then the player won't pick the coin.
I'm sure there are more optimized ways to do this, but given the time constraint and the low memory usage in graphics, I just went with what I think is the easy way!