1. Trump is articulating MORE Republican positions then MOST republicans in political power ever articulate. He's more republican than most republicans.
2. Yes, he attacked Scott Walker's success AFTER Scott Walker's campaign attacked him.
3. Trump got where he is in by beat the competition. He understands that to win the nomination he has to beat the competition. That's what he's doing.
Supporters and staffers regularly say critical thing against other candidates.
With Trump, and the new clipped-wing military (which it will take a decade to fix), The adage "Speak softly and carry a big stick" will have to be changed to "Carry a twig and wear a thin skin."