Good point about the yellow devil, I could still see it working, though. You could choose where to attack and when to switch sides.
Can't say anything about Metal Face, since we have no idea how that would work in Smash 4.
Of course playing as a boss would be an unfair advantage, that's the point. That's why you could be up to four against one, or three against one if you people don't want the gamepad to have anything special about it.
All that effort could have been put towards making Ridley playable, yes. But this wouldn't be a compromise, it would be a game design choice from the start. He's doing things with the 3DS version that's only possible on the 3DS, why not do things that's only possible on the Wii U?
It wouldn't make the Ridley fans happy, no. Making Takamaru an assist trophy doesn't make Takamaru fans happy, either. You could argue that there are more Ridley supporters than Takamaru supporters, but I'm not sure how much Sakurai really cares. Fans are fans and no matter what you're not going to make all the fans happy. Ridley has more supporters than Duck Hunt Dog, but what happened there? Sakurai does want Sakurai wants.