That word "forecastle" is extremely generic, and has been used to mean different things during different periods - and, I suppose, in different places. A nineteenth-century British or American merchant seaman might, for instance, call the place where his quarters were located the forecastle - even if it consisted of a deckhouse aft of the foremast. I imagine the crew of the Wasa used the term (or rather its Swedish equivalent) to refer in general to the forward, upper portion of the ship.
So far as I'm aware, there's no doubt that the Wasa's current deck configuration is the one she had originally. In modern English-language parlance she'd be labeled "flush decked" from the bow to the break of the quarterdeck. The Airfix kit certainly seems to match the ship.
I believe there has been some discussion about the original configuration of the bow - specifically the rather odd (to non-Scandinavian eyes) "bulge" above the beakhead, where the beakhead bulkhead would be in an English ship. As I understand it, that part of the ship was damaged when she was brought up (various people started bashing the wreck with grappling irons shortly after the sinking), and there's been some argument about how it looked originally.
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