…and saw a stranger approaching the same house, would you feel it appropriate to acknowledge that person in some way?
Done it many times. A vague “Hi”, without stopping walking, suffices.
Yes. A quick “G’day” or “Hello”.
Yes, I would do the same. Just a quick smile and a “hi” without stopping.
Sure, a friendly hello and a smile is always nice. 
Most certainly. But I guess it depends where the friend lives.
At the moment I’m living in rural Ireland and in the smaller villages you pretty much say hello to everyone you meet on the street. When I now go and visit London I smile at people on the Underground and they give me this “what you do want from me, you freak” kind of looks.
Depends on the stranger. If he has a big black cloak and a scythe I’d probably quickly find something really interesting to look at across the road.
I would just give the vague “hey”. But do a double take and then warn my friend that a “weirdo” is approaching your home via Nextel.
I would say either a “Hey” would be appropriate or a nod of recognition would be good too.
I’d probably walk casually by and nod or something, then when they were just past me I’d apply my elbow sharply to the base of the skull. If that didn’t drop them I’d do a foot sweep to knock them to the ground, perform the Kentucky Fried Movie patented Transmission Drop to the torso.
If they’re still alive then I’d question them profusely. If they failed to talk then, I’d send them to Detroit…
Oh, wait, sorry, that was a dream…
Stainless.