Here is a link to a prodecure you can use to make your hth kit read higher chlorine levels... http://www.poolforum.com/pf2/showthr...a-good-testkit)

If you lost that much chlorine overnight (assuming the 1 ppm reading was before sun hit the pool?) then you still have a good ways to go in killing off whatever is in your water. If it was after the sun hit the pool, then we don't know if it was due to oxidizing goo, or to sunlight. If you'll test at night and the next morning before the sun is on it, you can compare the numbers and get a better idea.

Your calcium level is not high enough that I would expect calcium scale, but I could be wrong--can you flake off a piece of it, put it in a covered jar with some vinegar overnight and see if it dissolves?

I would not use floc at this point--you need to make sure that you've killed off all the living algae, and filtered out what you can before you even go that route, since if not used carefully it can create other problems for you.

Regarding swimming, as long as the pool has chlorine in it and the pH is within range, the biggest danger is going to be swimmers underwater in cloudy water so that you can't see if one of them is in trouble or not. If you can't see the swimmers, I wouldn't let them in.