Three ways this could all turn out ok(ish):
1. Donald Tusk and his mates think that Boris is indeed mental enough to go ahead with no deal, and decide to capitulate on the WA in some way. Utterly unlikely to happen, they seem to have stronger principles than we do.
2. On October 29th, Boris addresses parliament, points out that whatever deal is now on the table is absolutely unarguably the best deal there will ever be, having pushed everything to the furthest brink imaginable, and then puts some options to parliament, possibly including no deal, the existing WA, withdrawing A50 or a binding public referendum. Ideally, he points out just what a nutjob you would actually need to be to want no deal, which coming from him might calm the riots a little. He then calls Farage a silly clot, and then implements the choice of parliament.
3. Someone, somewhere, negotiates an option to rejoin the EU at a later date. We have a few years of apocalypse, then come crawling back and serve as the warning to other members whose right-wing elements get twitchy.