Better with a magnet then than no magnet at all though surely?
If there is crap in there, at least you have more of a chance of it being held away from important parts.
Wouldn't any rubbish get trapped in the oil filter?
Motorsport developments do decent ones. Sadly they dont list one for the 172, but I would imagine its just cause there hasnt been a demand, I am sure that one would be available if required and someone knows the thread size etc.
Not if its fine metal particles, it only stops the bigger stuff.
Rub a strong magnet against your sump plug to turn it magnetic.