The upshot of it all is that I'm finally in a position to actually start building my game. To date it's just been infrastructure mostly, and there are still a bit left of course. But as you can see, the level editor is now gaining momentum.
Those are the 8 basic tiles I've modeled so far. There's plenty more to come, and they've been manually placed in XML because the editor is still lacking vital functions. Something else I've managed to accomplish is a solar model, or sunlight & moonlight model if you want. It manages itself through calls from the Update() method. The main game code does not have to do anything special to change the direction or the light color. It all happens within the instance itself. You simply instantiate it at a certain time of day and it starts ticking over. The effect as it goes from dawn to dusk through the color and direction changes is quite amazing to see. Of course, in the finished game, a complete cycle of 24 game time hours will probably take around 3 hours real time. It should be subtle, otherwise it will distract the player. However, if someone does play for 1.5 hours straight, the changes will be noticed. That's the sort of scenario I'm looking for.
And as you can see, I managed to solve the problem with the incorrect rendering of hidden faces. Some settings in the shaders solved it. Not sure what device state in XNA 4.0 is appropriate to fix it yet though.