Hi confusedman,
Thankfully I've only ever had to treat white spot with single (as per instructions) doses of medication so I'm a bit unsure what to suggest for the best.
I think if you've already used exit for 6 days it should have killed off the ich, assuming it was actually ich to begin with. Re using if its not 100% needed is only going to stress out the fish again.
You never said if the other fish developed spots, or you did and I missed it.
Is it possible to put the molly in the quarantine tank and try to treat it separately? If so maybe that would be best.
If you do end up with a definite outbreak of white spot then you'd either have to dose again or maybe try another type of treatment.