Not entirely sure you are right on this. There is one precedent for the DUP doing a U turn, when the gun was to their head of course. They said they'd never share power with Sinn Fein. They did, much to the disgust of people like Jim Allister.Anyone that thinks the DUP can come on board is simply ignorant of of the history and ethos of the DUP.
I suspect that the DUP will do another U turn if some face-saving word formula can be devised to alleviate their embarrassment, at least in the eyes of their own more gullible supporters, and if they know that enough of the ERG MPs weigh in behind them to get the deal over the line. The most memorable posturing about "blood red lines" is always going to be associated with Arlene Foster by the way, but the real DUP leader at Westminster is Dodds. He may calculate that Foster is already doomed as a result of the RHI Inquiry and that she shoulder the blame from party hardliners. She's a handy scapegoat.