Model contains no problems of the type you describe.
(you may have dynamic lighting issues later due to open meshes though)

I DLed the model, straight import to MED, saved, and loaded as entity
in WED into a pre-existing test level of mine, and set polygon flag on.

It works fine here, I cant get into the park cause of its wall, and cant
get into garage as there are no doors, but Im able to hug the walls
right the way around, and if I place my start inside its all cool.

The problem is somewhere else....

How are you adding the model to WED?
Object / Add_Model, Object / Load_Entity? (same thing)
or as a prefeb or terrain?