I certainly think a Forest might be giving us a clue to pronunciation as well as a bit of laugh, yes.

I think the idea is that it is very flukey that it lands on the step, and if Lawrie manages to stumble so her hand is just outside the window and simply lets go as she does--there's no implication it isn't a completely genuine accident, is there? Or am I missing something?

The knife does have another minor plot-significant moment in Autumn Term, yes, and I think is incidentally mentioned elsewhere too.