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Would you view the dio as incomplete without the ladder?
Not really. You can't fit a whole building on such small dio, so it may be possible your sniper went upstairs "out of the picture". Just make sure you don't place him in a position that cannot be accessed from somewhere outside the dio. To use the example picture you posted : it would NOT work without a ladder. If the first floor with the sniper would reach the edge of the dio, I don't see a problem.
If you don't like Spiderman, a superman, Zorro or Batman costume may make it believable too. Otherwise, leave traces of the UFO that dropped him.
And if all else fails... Would it be impossible for your sniper to be already in place BEFORE the rest of the building collapsed ?