My parents could keep that going for a good long time by filtering it through a few layers of kitchen paper, it got rid of a lot of the burnt stuff, came out quite clear each time.
That depends on the dish - a Wiener Schnitzel for example should be able to move freely in the oil (because it should be kept moving while frying), or the breading will be pretty underwhelming. Same goes for most stuff with breading. I always try to be very conservative in my cooking oil usage, but in those cases it's just not an option.
A lot of people do, actually. Seasoning them right for them to come out more "french fry" than "dried potato" is more involved than a lot of people might like, but I personally don't do it because I'm being picky about the flavor. I don't own a deep-frier and salted-dried-potatoes are good enough with a lot of entrees.