If worse comes to worse and Lego does end up implementing a "one promo per household" rule in all cases, just be prepared to adapt and move on. If there's one thing 16 years of selling on eBay has taught me, it's how to deal with change.
If they do that kind of limiting, they're basically just making those exclusives even more valuable, and at the end of the day all we as sellers end up doing is passing along our additional expense (in this case, not getting as many of the promos to resell to mitigate the cost of our other purchases) to our own customers down the line.
What's annoying, and I'm as tetchy about it as anyone, is to be in this current state of flux where they seem to be trying to play the "oh, our crystal ball says you're a leper - I mean reseller!, GOTCHA!" game or they're just incompetent at implementing and standardizing a new policy, or both.
Presumably they will get that ironed out and start applying the same algorithm to everyone and we'll all at least know or be able to make better guesses at what the "real limits" are, and from there we decide what to buy, when to buy, how to buy, how many hoops we want to jump through to game the system, and how much we have to increase our prices to cover our cost of goods sold.