If I read you correctly, the reason a very low powered light source is used is to provide a gap (gap of time, any length produces the same effect) between each particle passing through the slits so as to render any actual interaction between them impossible. The word probablilty is often used when it need not be. My recollection (which could be wrong) was that no particles went to the wrong part of the diffraction pattern at all. That its construction was totally ordered.