1) That depends on the temperature and relative humidity, the surrounding airmass, and the overlying airmass. It also depends on the type of aircraft surface, part of structure, texture, preventative measures, etc. It also depends on terrain beneath. I've had some really unpleasant icing down low, and some very unpleasant icing up high. It can occur above 40,000 in visibile moisture, especially near convective activity or orographically lifted air. The worst icing I've encountered has been between 9,000 and 16,000'.
2) I use the payless index. (Walmart index, when casual). I step outside and walk, and if I don't fall on my butt right away, it's not too bad. If I do fall on my butt right away, I fall back to the Sawnut index, which first determines if anybody saw nuthing. If nobody saw anything, it's still not bad. If anybody saw me fall down, I get up and make sure it doesn't happen again. If it's really bad, I resort to th cleat index, occasionally tempered with the cat gravel or salt rock index, and confirm the issue with neosporin and a bandaid. As far as landing...who cares? I'm landing.
3) The clean aircraft concept is A) unknown among normal pilots, but expected by all passengers, and involves no cracker crumbs in the seat rails, a full compliment of airsick bags, and an armed pilot to shoot those foolish enough to track cat hair or dog hair on their airplane.
B) the clean aircraft concept involves keeping aircraft surfaces free of contaminants which might otherwise degrade flying capabilities or performance. It involves ensuring that frost, ice, snow, and other contaminants are not fouling lifting or other flying surfaces, and it also involves a thorough understanding by the PIC of icing, deicing, and prevention. It requires adherence to inspection proceedures, and to taking responsibility for determining that the aircraft is properly cleaned, kept clean, and ready to fly.
A good reference to read on the clean aircraft concept is found at:
http://www.faa.gov/avr/news/Clean.htm