OK.
The magnifier itself can't fail from overpressurization; it's a solid piece of sapphire or acrylic. Nor does it have any effect on the watch's pressure resistance. You can easily test this yourself by taking two Sea-Dwellers — identical in all respects except that one has had Rolex's magnifier glued on — and placing them in a pressure testing chamber, then noting the point at which they fail. Note that you'll need a rather stout apparatus for this to work properly. Repeat as many times as necessary to get a statistically valid result, and you'll find that the presence of a cyclops (or even a dozen cyclopes and a cyanide capsule) on the crystal makes absolutely no difference.
(You could also demonstrate this fully-understood property of fluid mechanics by using cheaper materials, such as ping-pong balls with and without wads of chewing gum affixed, but where's the fun in that?)