Yeah. The whole point of the Adelsons buying the Mavs was to make it the centerpiece of a new casino development in Dallas which is a potentially huge untapped market. Buying an NBA team just to move them to Vegas so you can open your casino seems kind of pointless. If they wanted to do that, then try and convince the NBA to expand to Vegas as you mentioned. I'm also too cynical to think that they are going to perpetually have trouble legalizing gambling in Texas as well. If the right people want it, and the politicians start to understand the amount of money to flow into government coffers from it, it'll get legalized.
Minus the "move the Mavs to Vegas" part, what is laid out there actually sounds perfectly reasonable to me though. There is a lot of online chatter about it, but not a whole lot of mainstream media chatter about it (Bill Simmons is the only major media figure I've seen mention it). The most logical explanation for the trade is that it wasn't a basketball decision, or at least wasn't purely a basketball decision.