I've cultured it a couple of times without problems. I used it in a lighter (colored) ale and it was not bad. Pretty funky with a lot of Belgian phenolics and esters, even when fermented in the upper 60's. Also, from what I've read, Hennepin, Dubbel, etc from Ommegang are all the same yeast. I've also read that it isn't but I can tell you from experience, it is pretty close if it isn't the same.
I can't comment on the Duvel yeast vs. the El Diablo strain side-by-side other than to say that 1388 from Wyeast and bottle-cultured Duvel yeast side by side are more or less identical and that Duvel yeast seems to be a lot "milder" than a lot of other Belgian yeasts. Also, I've had luck culturing Flying Dog's yeast in the past so it shouldn't be too hard to get a culture going.
So far as creating a culture, I'm sure you've read about the procedure so I won't bore you. You are hitting the nail on the head regarding drinking more than 1 beer and combining it into a single container; the numbers just add up to a more successful culture if you've got more potentially viable yeast. If I don't drink all of them at once, I put the dregs back into the fridge. I've been known to also make a small starter with a single beer and then pour more dregs on top of it as I drink them and gotten good results that way, as well. It isn't rocket science but it does involve luck, sometimes, with some strains of yeast.
Hope that helps and good luck!