Hi Nathan, sorry for the late reply, Im waiting to have it to reply.
Ok, the tangents do look correct. I think the problem is that you need to find the osculating points on the curved path fillets. There is a toy for this (called fillet-minion). See if that makes for nicer fillets.
You are ok, oscilating point is wrong. I surf into the 2GEOM toy filet-mignon, i do about but it give me the strange results pointed by Cale Gibbard in the #3 of this launchpad bug https://bugs.launchpad.net/inkscape/+bug/202751 Finaly i adopted the Cale solution in the same bug https://launchpadlibrarian.net/12692602/rcp.svg
Soon I comit the new code. I have a extra problem related to time values on curves, I find that not straight lines have his time values based by his handles instead the distance along it. My cuestion is: Any way to know a point at a distance based time value?
Sorry if not explain so well. Regards, Jabier.