So, upon further testing (and this should clear some confusion): dunno why, but the Toriel mod doesn't work. At all. Just like it happened to you, did the same thing to me, and this was on a fresh install of the Steam build. My guess is that Mozu didn't consider the changes it would bring when tweaking the game for Steam but not for the mod. Therefore, the images aren't compatible, the game refuses to load, and it just crashes. That, or the mod is an older version and needs to be refreshed. I'll be getting a new copy through their Ci-en and try again, Meanwhile I'll also be trying to see if the same thing happens on an older build of the game before the Steam release at the cost of the mosaic censorship. Small price to pay however.
EDIT: Seems I was right. Grabbed an older build from svscomics, waited like an hour for it to download, works almost perfectly. The only caveat is that the text is a tad glitchy, but the rest of the game runs fine. It's also version 1.3, probs the one before the big Steam release. I highly doubt that Mozu will update it since it's fairly obscure, and if I am indeed wrong, I haven't found a fix or an updated version of said mod. If anyone wants that build alongside the Toriel mod, lemme know.