It seems you encountered 2 problems:
1 Fillet has a bug
2 Extend curve actually adds a new segment, so the fillet functions stops at the junction of new segment. You can fix that by using “Simplify Curve” component
An idea, instead of constant fillet radius, would be to set the fillet radius as a function of the length of the two side lines. something like this sequence: for each curve, explode, then curve length, then get the smallest length and set radius equal to, say, 1/3 of this length.
well, if that approach is not working, you may have to think of some different logic. Possibilities include using discontinuities and midpoints as source points, then interpolate curves. Also, after you have produced your curves by whatever means, you can look for curves that are not what you want, and flag them for even manual work in Rhino, for example look for open curves, curves that have less segments than the original veronai cell, and so on.
Another approach is that you have Grasshopper do most of the work for you, then you manually, in rhino, finish your design.
Stick with it though, the ideas do not happen all at once, more like incremental improvements
Thank you @markz I think I kind of solved it. Not exactly what I wanted , but good enough at a masterplan scale.
Not perfect, I might come back to this problem…