Cool find and a very interesting analysis!

There's a lot more to morphology than just the shape of the shell, and indeed the shape can sometimes be misleading, in that very different species can have somewhat similar shells, and different individuals of the same species can have quite different shell shapes. You've got a gasteropod, so it would be good to pay special attention to the peristome and siphonal canal (based on the bio classes I took in the area, I'm no expert) but of course there's lots of features that could be helpful in an identification.

https://en.wikipedia.org/wiki/Gastropod_shell#Parts_of_the_s... is a good list, and maybe you've already done this but you would want to find a dichotomous key of gasteropod families native to the area to narrow it down. Good luck in figuring out your shell!