Were you surprised when you realized that it's pay 2 win? I mean, isn't it pretty obvious? Ofcourse it's pay to win, if you can buy stuff like shadowmourne.
Your post sounds like it has little to do with multiboxing to be honest. You're annoyed, because you lost to a premade (in this case a multiboxer), if it was a premade of 5 players (or however many characters the multiboxer had) you would've lost even harder. It sounds like you're annoyed that your pug lost to a premade, but I have no idea why, isn't it normal that a pug loses against a premade?
The 2 main "issues" that pop in your post is Donor p2w stuff and multiboxing, which is why I suggest that you reroll to Lordearon, there's not that many multiboxers there and you can't purchase BiS gear from shop there either.
And yes, multiboxers have good teamwork, but they don't have as good control as singlebox-premade. It has always been Communication vs. Control when talking about multiboxing and if it's fair or not. If you look at it closely and think it through, you will most likely end up with the conclusion that single-box premade will have the advantage. I'll give you an example: Multiboxer will never be spread out all over the BG, but a premade can callout targets and improve their communication to near perfection.