Re: Wax Moth Larva
Yes, do exactly as you say. Either put something on the ground to put the super on while you inspect the lower one, or else put the super on the ground on it's end, not on it's side.
The normal procedure is before opening the hive give them a smoke at the entrance. Then take the lid off and smoke the top of the hive. Then put your hive tool between the two boxes and prise them apart. Puff some smoke between the two boxes, then lift the top box off, putting it on something, or else on it's end on the ground. You may then inspect the whole hive.
Re the wax moth, you describe it as big, so it's likely a greater wax moth. Long as the hive is strong it will deal with wax moths, but a weak hive, or one with unprotected comb, can be overun by them. No need to panic most hives get the odd wax moth hanging around.
"Thinking Inside The Box"