I've got a total of 3 DS50s, which haven't changed the DS51 in terms of the battery door seal, and none of them have ever flooded in my posession. All three were bought used. One (the oldest, both in my ownership and in serial number) does show some damage to the battery orientation label that indicates there was wetness inside the battery compartment at one point or another...but that's it. When you first submerge them you might see a few bubbles as the channel around the door itself floods (with surface tension it might hold bubbles inside for a bit), but that should be over very quickly.
A few suggestions:
DO clean the o-ring mating channel on the strobe side. With face-sealing o-rings this is just as important as keeping the o-ring exposed face clean itself. Excess grease can build up in an apparent hard surface but will still be porous (or worse yet attract sand, etc.)
DON'T overgrease these o-rings. They're face-sealing so you don't need 'lubrication' to allow mating surfaces to slip over them...just enough to keep them the slightest bit glossy will do. Any more actually does more harm than good, since it breaks the surface tension of water, and potentially lets debris (hair, etc) stick to the ring.
DON"T overtighten. Hand-tighten as much as you can (unless you're the Hulk, in which case back off even from that) and then at most another quarter-turn with a coin. Under water it's water pressure that holds the seal anyway, the screw is really just for the surface.
DON"T leave them out in the sun, or put batteries in when hot and freshly charged, or otherwise do something that could lead to too much hot air pushing its way out past the o-ring and leaving too much of a negative pressure interior vs. more normal ambient temperatures. Once near the surface in cooler surroundings, they might 'suck' water past the seal.
DON'T use any batteries with any damage to them. A single drop to the ground can 'dent' the negative or positive cylindrical end to the point where the batteries don't make good contact and the strobe won't work, and/or might also somehow contribute to the door not pushing down flat. I did have this problem (in the sense that the strobe wouldn't fire with a particular battery or two, which were just not seating properly). There's really no excess space in that compartment at all.
I've only got a sum total of about 35 dives on these three strobes - 15 on the first one and 20 on the newer set of two - but I think I can say your experiences seem like really bad odds. I'd have thought if the design overall was that bad, with 3 old ones I'd have seen at least one failure by now. Or maybe it's a matter of 'infant mortality' and I'm sampling data only from "survivors"? 
