OK so what if Walker is the nominee and Trump runs third party? Who gets hurt the most in that scenario?
In that scenario, Walker should work to earn the votes of Trump supporters. Those votes don't belong to him nor to the GOP, and I would still say that blaming a third party spoiler is just a way of a poor candidate passing the buck.
As to my own votes in a 3-way race between GOP Nominee, Hillary, and Trump (assuming Trump isn't the nominee, and ignoring 1% zero-chance candidates):
GOP Nominee = Cruz, I would vote for Cruz.
GOP Nominee = Walker, Paul, Carson. I would probably hold my nose and vote for the GOP nominee.
GOP Nominee = Bush, Rubio, Huckabee, Christie, Kasich, etc. I would probably either stay home or vote for Trump.