:

Yep, exactly.

I only knew to go past her this way only because I played the demo previously, in which I did do and got the “You felt bad and so you went back, didn’t you?”

I have to admit that this is true for me too. I mentioned that it was obvious but I probably would have missed it.

The only reason I knew you could spare her is because I played the demo years back. 

Unless you see that Toriel is holding back, that doesn’t become obvious. Which suggests that people who are actually good at dodging would never see her restrain herself and would just keep attacking like normal.

And I remember that after trying to spare her several times the ellipsis response comes up a few times. So I just assumed that like the talk option, there was no way it would progress.