-so how did she find out that he didn't actually want to go through with it before she met Asgore again after so long, when he's likely never shared that fact with anyone besides his closest friends, like Gerson?
And if she did know, then her "take one SOUL and go to the surface to take the rest" plan wouldn't make much sense, because even if Asgore tried taking the other six SOULs through less violent means, the humans might still be on edge about monsters after they thought Asriel had killed one some time ago. So, them seeing another monster (the king of monsterkind, no less!) who looks strikingly similar to the previous monster who entered the surface, already bearing one human SOUL that would give him a frightening amount of power (as well as imply that he killed a human to get it), politely asking for six more humans to pretty-please give up their SOULs so that he'll become an unstoppable godlike being who could ravage the entire world, just so that he can break the barrier keeping his people trapped and totally not use it for other, more nefarious means... Yeah, it likely wouldn't end very peacefully, and Asgore would either end up like his son, or causing more bloodshed, like he was afraid of.
In short, Toriel's "take one SOUL" plan would only work if she thought that Asgore was on board with hurting and killing more humans, which again, begs the question as to why she'd say that Asgore was "hoping that a human would never come." My only other idea is that she was assuming that he was prolonging his plans out of some form of cowardice or incompetence, rather than straight-up reluctance.
So uh yeah in shorter, local guy with Asperger's is confused about a single piece of dialogue from a Mimiga ripoff (/j) in a game that he hyperfixates about but rarely plays