But if backbenchers managed to form, say, a national unity gov't within the two weeks after the vonc, then HM has to decide to either sack Boris and appoint a new PM; or refuse to appoint the newly elected leader of the national unity gov't as PM whilst allowing Boris to continue despite having lost a vonc. So it will be unavoidable for HM; either choice forces her to meddle in politics.
Edit: I think it would be ridiculous, if not inconcievable, for Boris to hang on if backbenchers could form an alternative gov't once he had lost a vonc. What are the future prospects of a PM who basically squats to hang on to power? So I think he would probably walk in that scenario.
He might hang, though, if there is no alternative gov't formed after losing the vonc.
Or, he might even survive the vonc.