This thread is very old and Rokas doesn't work at iToo Software anymore.
I checked your posts, but i'm not sure if i understand the problem correctly.
As far as i can see, you want to control where to use a Corner segment, setting the vertices as Bezier-Corner.
Unfortunately that is not not possible when using 'Fillet', because the fillet function modifies the original spline, creating new BC vertices in the process.
One alternative if to control Corner segments using the spline Material ID, as shown in the attached image.
Basically i assigned Mat ID = 2 to the spline segment BEFORE each corner.
Please tell us if that may work for you.
I have not uploaded the scene because now we use Max 2024, and i don't know what Max version do you need.