What I meant is that, as you wrote, seers etc were very influential even in our world, were magic (as intended in a standard fantasy rpg) does not exist. And this means that, in a world were magic is common, a PC able to cast Death's Tale True will very probably be trusted by local authorities (he is a high level cleric, he should be quite influential...).
But no more than he'd be in our real world, where magic doesn't exist and where, back at such times, people
did believe it existed and was "common" (may it be miracles, or witchcraft). So, once again, I don't see how the existence of such magic would drastically change anything from playing a game set in our RL medieval time.
Also, being influential is a relative thing. Of course, if as a 19th level Cleric (
Death's Tale True), he's being investigating the murder of a local farmer, I'd expect the other peasants and the local authorities to believe him, but I'd generally expect his opponents to be about the same political level than him, meaning just as influential, so it'd still be his word against the other person's word --therefore still posing the problem of finding more "concrete" proofs.
But the real problem is that the players know that the spell tells the truth (we're talking about a game so we shouldn't forget about the people around the table!) and, unless you reword it, explaining clearely what it can and cannot show, as Rasyr did, they'll know what the solution to the mystery is.
Well, I'd believe that, back in the dark ages in our RL world, most of the wizards, seers, astrologers, witches and the rest did believe whatever means they used to see things told them the truth as well...
Frankly, I think the best way to deal with such spells is to give the other side access to equal means, meaning spells to misdirect the scrying. Not always, just sometimes, in a calculated manner, so that whenever a character uses such a spell, the player would still wonder whether he wasn't misdirected
this time... It's the way I manage it with one of my PCs, who is a Sleuth (RoCoIII).
Another may be to manage it a bit like
Teleportation, with time since death being the deciding factor about how accurate the vision is.