... utilizing 3 boards each, stacked in such a way that there is a total of 5 forest rows and 5 "cave" rows...
So it is widened? And the central area is 3 pieces thick, while the ends are 1 thick? Can you post a picture?
Why this particular expansion? I can see how having 3 dwarves would get you into the mid-game sooner, like my own Quick-Start Learning Game, posted earlier.
Why did you feel that the forest & cave areas needed expansion? It's kind of like giving every player the Office Room (without the gold bonus). I wonder if a 3-player group would enjoy this, too.