Your plan sounds good to me...if you're holding chlorine overnight and have no abnormal consumption during the day, and if your water is back to blue, I would just hold the Cl a little high until your party is over, and shock afterward just to kill any remaining "nasties". I think the film that you originally found on the bottom and sides was residual live algae, which can be present with no CC reading or huge drop in chlorine. I also think that since the film disappeared and the deposits appeared after an extended shock, that you probably had deposits of dead algae on the bottom and correctly vaccuumed them to waste.

I strongly suspect that you're either fighting pollen (don't remember offhand where you are but we're fighting an ongoing battle with it in Louisiana this summr), or possibly some sand blowback from your filter. Is there any other way that dirt might be introduced into the pool? Usually when the deposits start showing up in the same places, especially if they are near the return jets, it's more a matter of dirt or pollen that is settling there due to water movement. Algae usually deposits itself a little more randomly, and takes up larger spaces.

Janet