Also:

I was thinking the 'S' button for enter/exit might get in the way, but then again, a lot of platformers use a mechanic like that... We'll just have to see, I suppose. Having the controls in one familiar area is a benefit to the player (who, we must assume, does not want to spend time learning our specific control scheme).

My reasoning for 'Spacebar' as fire (when walking around) was this:
Depending on what Tom wants to do, we may or may not have a mouse-aiming mechanism for the walking character. Having 'Spacebar' as fire when walking makes sense if there is no mouse aim, as it'd be the cannon's fire button and the fewer the controls (and the less they change) the better.

Not having mouse aim, I feel, would make the walking character feel more vulnerable... Which is what we want. We want the player to be wary of exiting the vehicle. If they can fire all over the place all the time, jump over obstacles, and have their own light source while walking, there aren't nearly as many reasons to stay in the clunky carriage- In fact, some areas might be easier without it (which is something we desperately want to avoid).

So yeah: I think the player, while walking, should only be able to fire in front of them. I think climbing is a better idea than jumping (much creepier, more physical, and time-consuming- making the player feel more vulnerable), though it may be much more difficult to implement. So we'll get what we can out of that.