There should have been cases of someone delivering a message or performing a rescue. There should have been researchers ambitious enough to be able to join top-tier adventurers.
My point is that the desperate rambling of some youth alone shouldn't have been enough as convincing evidence. It wouldn't have convinced me to let them go back into the dungeon with that alone. There were either cases before already and Yoke simply confirms it, or the theory never actually mattered and he was let go even if he was talking out of his ass. The way the manga depicted it, it didn't suspend disbelief.
No there shouldn't have been cases of someone delivering a message; it's the final dungeon, the biggest, hardest, most dangerous one. While people are in there, they are incommunicado, because they can't just send random messenger boys down after them in good conscience, not even if they were decent adventurers themselves (or rather, especially if they were decent adventurers themselves).
Similarly, researcher generally means they're poor adventurer because of the focus on knowledge, and most would end up dead from curiosity and not properly adventuring, unless they're being dragged along by a strong party. Furthermore, we know of only four parties in history that have gone into the dungeon and returned at all; the one that his parents were part of that eventually vanished, the other current supreme party, after the unsealing, and then his group and thunderpike together
as part of the process of trying to figure out what the heck is going on with the dungeon. Yuke is basically the closest thing that they have to a researcher-adventurer, because of his quick thinking and generally high intelligence.
Lastly, the explanation to other people mattered only in that it provided them an excuse to allow them to re-enter because it was acknowledged that without permission, they were desperate enough to force their way back in anyways. It was sufficient to be something to check for as a justification. The concept itself was still something that matched up with how events unfolded along with other information gathered until then. Simon also wasn't speaking out of desperation when he was talking about the whole being able to find them bit. If you still can't understand that, that's fine, that's on you, but enough already of willfully refusing to understand that this world is not modern day with its core scientific knowledge, principles, and analysis methods.