Well
Spiky Bugger you are right, but your analysis left out a couple factors.
First, with gastric bypass, the minimal malabsorption is certainly a factor, but along with that in some people is the loss of restriction if/when the stoma enlarges over time. Once that happens, it doesn't matter whether the pouch is large or small, whatever the person eats just falls right out of the pouch into the small intestine because there is no valve to prevent this. This leads to a sensation of hunger and for some, also hypoglycemic episodes.
With the Ds, I agree that a diet high in carbs will defeat it, as it would any bariatric surgery. And while booze and soft drinks can certainly contribute, any simple carbs will do - junk food, fast food, whatever. An increase in issues with alcohol has been documented with gastric bypass, I don't know that this is such a problem with the DS - haven't seen it, but that could be because it's not a problem OR because it hasn't been studied in the DS population.
I also agree that a return to a DS healthy high protein, low carb diet should work to reverse the weight regain with the DS.
So, you were mostly right, but I do so enjoy correcting you!