I don't think there are any good answers to this one but, IMV, stripping her of her citizenship was not a good move and is not, very generally speaking, a good route forwards.
Ultimately, I think probably the best route forwards would be to make a show of repatriating all of our former IS fighters/sympathisers and then to put as many as possible on trial for treason in a very public way. Perhaps even consider implementing the death penalty or, at the very least, whole life tariffs.
That said, I do think the most likely outcome re: Begum is that she would be repatriated, would probably escape any harsh punishment (perhaps a handful of years in some relatively comfortable women's prison) and then would be a major cause celebre for certain sections of British political Islam. But that gets at something much, much deeper than this case - a genuine clash of civilizational outlooks which is likely to play out at many different levels over the course of rest of our lives. Multiculturalism/pluralism sometimes fails and this is an area where it is in serious trouble.