Surely there must come a point where it is economically viable for Heathrow to offer to purchase all the houses in the surrounding area (at 20% premium on the market rate, say) meaning that there will be no further objections from residents, because all the ones who don't like it will have sold up, and those who have chosen to stay obviously aren't bothered enough by the increase in noise another couple of runways would bring?!
Not really. There are two entirely separate challenges - finding land on which to lay down the concrete for a new runway. Unlike a few infamous motorway diverts, where single houses have been built around, the runway is all or nothing, so the compulsory purchase system would be the only option.
The other issue is noise, and I suggest people fall roughly into 3 camps - people who work at or are heavy users of the airport, who accept the noise, people who are seriously bothered by it, and then a middle group, who aren't that fussed.
So the only you might be able to buy out some of the latter 2 groups, but to what end? You'd be left with a patchwork of houses with no real continuity. So all you could do is insulate them heavily and sell them on, but it is much cheaper just to offer an insulation grant scheme, which I'm sure will come with a third runway as one of the many planning conditions.
As for Surrey, it is quite common for major schemes to cross two admin boundaries, and not unheard of for them to take very different views (we had this at CVT for a 2m pax pa terminal!), but either way, this would have to be determined by an even longer public inquiry than the one that approved T5, unless it was done by hybrid bill / Act of Parliament - not sure about the technicalities of that, it has historically been used more for railways.