Ah you're not stuck at all my friend, that door is locked out in the demo! I still haven't implemented what's behind it...but it is very important to the game's plot. Basically, at some point in the game you may find a way to get those broken doors fixed...
The invasion allows you to unlock the flintlock if you haven't killed the Red Mother and you help her clean out the settlement. I can't remember if there's anything else exclusive to it aside from fighting more enemies, though getting to Wilbur is easy as the blue guys near his entrance will disappear.
Maybe you've done all there is to do in the demo? There's three bosses in all. If you want to reset the game and keep your items, use the rope wheel to leave the settlement.