Kamelot DID use overdubs in backing vocals.
I was watching the DVD for the 28th time (

) and it's clear to me, they used it in "Forever" chorus. The second and third time the chorus is sung, it goes like this:
will you revive
from the chaos in my mind
where we still are bound together (aaaw toogether)
That "aaw together" backing vocal is CLEARLY Khan's voice, when he is actually singing the "bound together" part of the chorus.
Regarding "When The Lights Go Down", I agree with Rad, I think the chorus is sung slightly different from the CD, sounds like the backing vocal people they put there.
Anyway, amazing DVD. Only Maiden's Rock In Rio beats it IMO.