Hi mdesign, that particular area of 123D was not based on an open source library, it used Autodesk's proprietary Shape Manager geometry kernel. Shape Manager is a fork of the also proprietary ACIS geometry kernel (https://en.wikipedia.org/wiki/ShapeManager).

So no it is not possible for it to be used in MoI. Autodesk does not license Shape Manager for use in non-Autodesk apps. ACIS is available to be licensed but it's pretty expensive, it's more oriented around higher cost "Enterprise" level software. It could be possible that MoI would use ACIS at some point but sort of as a last resort.

Robust filleting does not really come from implementing a single algorithm well, it's about recognizing and handling a lot of different kinds of special cases for different corner juncture conditions. It takes many many man-years of effort to produce a robust one.

- Michael