The only way sand is going to get in your camera is when you switch lenses while the sand is airborne. No other way except through that big gaping hole where the lens mount is.
Nope. Every lens, especially external zooms, has venting holes, as well as the camera bodies.
The vents have membranes over them that prevent the ingress of dust and moisture.
Even if the cameras and lenses are sealed, using zooms or external focusing lenses increases the risks for sucking in dust and moisture. Olympus is no exception.
You won't pull in dust or moisture, let alone sand, if your body and lenses are sealed.
That has absolutely nothing to do with weather sealing, and it is not the camera's fault.
It has. If the sealing is worn out, old or damaged than even more dust can get inside the camera. Remember, it is just weather sealing, not water resistant, air tight sealing.
It
is water resistant
No, it's not. This is totally wrong information. It is
NOT water resistant. At best, weather resistant.
-- it is not water pressure rated. Rain can't get in, but if you immerse the equipment it may leak (and it certainly will if the immersion is significant and/or prolonged).
Yes, there are supposed to be gaskets, though the membranes and valves are
not documented, and in any case, membranes, valves and gaskets wear out over time, so using an old lens, like his 50-200 is suspected to be, is always increasing the risks and is actually a false protection. The 50-200 is probably one of the worse lenses, since it is very heavy, also the front part, and is extending that heavy front quite a bit when you zoom. Also since the front is not rotating the strain on the gaskets is always on the same points, not distributed evenly or randomly, meaning that the gaskets will wear out sooner on some places than it would if the front would rotate. If the lens is often used (he is shooting weddings, so I suppose he uses it often) than the risks are increased even more. I suspect he had that lens ever since he bought the E-1, so it can be as old as 7-8 years now.
Anyway, the manual of E-3 was full of warning about not using the gear under humid and dusty conditions, and that doing so would void the warranty. This is the first time I hear that Oly actually refused warranty based on that. If the same warnings are also in the E-5 manual than I don't think he has a chance, and it may show a trend that Oly service got tired of all the people claiming warranty for every abuse of the camera. Too bad if a person who has not abused his gear is one of the first to be punished by this new policy.
In this case, it sounds certain that the OP allowed sand into the camera during a lens change. It pays to be paranoid about lens changes -- you should be able to change lenses in a bag if you are in a very hostile environment.
Lens changing is always risky, but I am pretty sure the OP is well aware of that, he seems to be experienced and I don't have any reason to suspect that he was careless. Of course, I was not there, I have not seen him changing lens and have no idea how he handles his gear but I assume that he knows how to change lens. Of course, you are right, it most probably has happened during a lens change, but
could have happened even during use. There is no way to tell with 100% certainty.