Now that I'm on my desktop, I realize the GR2ConverterDAE indeed has a V1.9.0 folder inside it. That seems to be Norbyte's GR2 Converter then. Thanks for that!
Unfortunately, still no dice. The DAE created from this program has an issue with that exact same jaw bone, this time visible even in 3DS Max.
I've also just learned that this seems to be a problem exclusive to the Seraphim, or at least a problem that doesn't happen to every character. I just converted a High Elf to SMD and back to GR2 from 3DS Max, no issues whatsoever. Additionally, during some of my conversion attempts with the various tools, the DOS prompts would display bone scales for the models. Whereas the High Elf had only two bone scales applied to it, the Seraphim had scales applied to most if not all her bones, and the scale numbers for the jaw bone were in fact very abnormal looking, with a value of 4 point something whereas every other scale was around 1. It's clear then that the issue is happening during the conversion from GR2, not with the exporting from Max.
Unfortunately I'm not sure I have the know-how to do anything about this. Unless I get a converter that properly gets me a Seraphim model with no jaw issues, or learn how to use one of these so that the issue doesn't happen, I'll have to stick to my workaround of giving her jaw vertices to her head bone instead. She won't be able to move her mouth, but at least there will be no distortion.