I'm not sure if you're still considering this, but
here is a good mouse that I think might have enough buttons to play. I can't say for sure though. Let me think..
- Left mouse button
- Right mouse button
- Three thumb buttons - two of which are perfect for forward/back
- Mousewheel + middle mouse button (I don't remember if this was ever used in the game..?
- Mouse wheel left/right
- Two extra buttons for the finger (sensitivity control by default)
All of these except for the left mouse button can be reconfigured exactly how you want them, and you can make a seperate profile for Penumbra (as well as any other games you might play or programs you use). It can activate it automatically.
It's not exactly cheap, but trust me, it's good.