Cranfield was closed all day yesterday for a piece of high risk testing on runway 03-21 (water ingress testing on a prototype business jet so far as I could tell). That's not normally a quiet spot either. Even I thought a 36 hour closure was a little extreme, but wasn't involved - just happened to be doing something else on the same airfield, and nosey.
My first-flights have mostly been at public airfields, but all of them I managed to negotiate a fair bit of "elbow room" - in one case at a very busy military airfield, who were good enough to give us a short (but long enough) into-wind runway and leave all the jet traffic on a longer crosswind runway. Ask nicely, explain your issues, and it's amazing what you can get.
No, I don't think that Boeing are a bunch of amateurs by any means, nor necessarily are the management of Paine Field. A quick look at a plate suggests that the 748 was lined up on 34L, and those Cessnas could easily have been using either 34R or even 29, at minimal trouble to anybody. US airports are generally very good at juggling multiple runways. Probably more likely than my original suggestion of incompetence, or the OP's suggestion that they were all sharing the same runway.
G