Re: building furniture in vehicles. The terrain system and the vehicle parts systems are currently completely separate, the only way they interact is that if a vehicle is present the vehicle parts take precedence over whatever terrain is present.
As a side-effect of this, any “furniture” built in vehicles has to be defined separately from the normal version of the furniture.
I was looking at this just recently when I was modding in a schoolbus, I wanted the aisle down the center to not count as difficult terrain, but it seems that’ll require surgery in various parts of the vehicle code, so I put it on hold for the moment.
Just to repeat, having your vehicle keep going while you walk around in it would likely be quite difficult to implement and would touch some very prone-to-break-badly code, so even though I agree it’s a cool idea, I’m not going to be looking into it.