I don't have specific reservations about NBH as such, just not convinced that it offers anything that the good Islington secondaries don't.
Having said that, the schools you cite as likely to be offered are not ones that are generally considered sought-after, or that traditionally have a significant m/c intake. That doesn't of course mean that they are bad schools, but you would need to have a close look to assess whether you think they could work for your child in terms of meeting his social and academic needs.
Remember that you are not bound by borough boundaries, so worth looking at schools in surrounding boroughs. Acland Burghley is a Camden school but very close to the Islington border - it's had a rocky few years, but has traditionally been a good school that is popular with local parents, so maybe worth a look. Alternatives might be Haggerston, Hackney New School (or whatever it's called), Petchey Academy and Mossbourne Victoria Park, all of which are in Hackney.
And yes, there's loads of movement from the start of Y7 onwards as people relocate, and lots of those places are not filled, even in popular schools as people don't want to move once their dc are settled. So def worth going on the waiting list for all the schools you like the look of.