There is no such thing as “unstoppable”. Remember, this same chattering class thought the 2008 election was going to be a nail-biter between Hillary and Rudy. How’d that turn out, again?
However, Walker might not be the ideal conservative candidate (flip-floppiness on amnesty and ethanol are.... worrisome, to say the least), but I’d say he really ought to be considered the early favorite. He’s clearly a winner - something the GOP desperately needs - and he’s shown a determination to get key policies implemented in the face of heavy opposition and media pressure - something conservatives should be licking their chops over.
But invincible? No, not even close. Heck, the establishment wing is still trying to find a viable alternative (settling on Rubio?) and he’ll face pressure from the right. And there’s always a chance for a major unforced error, especially with a hostile media ready to turn any little thing, real or imagined, into a major scandal.
The only people who are claiming Walker flip-flopped are liberal media or GOP-e commentators.
Walker has flatly said "no amnesty" and has said he wants to end the ethanol subsidies.
He described both positions in great detail that allowed some to select sentences that seem to be a change in position. That is not true.
I suggest you read unedited transcripts of what Walker says and you will see he is thoughtful, clever and articulate.