During an LPV or GLS final approach, can the extension of the aircraft's cross-section line intersect with the Y or X OCS surfaces?
The OCS is the edge of the safe flight 'corridor.' It defines a safe volume of airspace for the approach. Since it's normal to have obstacles off to the side of the approach path the OCS typically rises as you move away from the approach centerline.
Your Case 1 is the most correct description of an aircraft flying the approach. The green 'floor' normally will extend greater than one 'dot' on either side of the localizer (or localizer look-alike) and is at least one 'dot' below the glide path. That is the reason a missed approach should be executed if you deviate more than one dot from the approach path. At the zero deviation level for glide path (your blue dashed line in Case 1 assuming the a/c is on path) the intersection with the yellow (x surface) should be beyond the full scale left/right deviation.
Essentially the OCS defines a canyon that you can safely fly through to the runway. So yes, the extension of the aircraft's cross section line can intersect the x and y surfaces, but the aircraft itself should not.