If it’s plausible for him to be burnt by a rebounding killing curse, then the evidence for a faked death is weak. If it’s implausible, he’d have found a better method to fake his death.
So it’s the second one. It’s implausible for a killing curse to rebound, so nobody would believe he died that way. Or at least, their less likely to believe it than believe a more plausible death.
If it’s plausible for him to be burnt by a rebounding killing curse, then the evidence for a faked death is weak. If it’s implausible, he’d have found a better method to fake his death.
It does not happen, there are no antecedents, nobody knows what happens.
So it’s the second one. It’s implausible for a killing curse to rebound, so nobody would believe he died that way. Or at least, their less likely to believe it than believe a more plausible death.