I made the offset_cubic function more stable and fast and merged it into
the latest version from Inkscape trunk so you can test extrapolated
joins etc., you can find it here so you can test it if you want:
I played a little with the Join Type LPE and collect path data samples
so I can automatically compare different offset methods:
Feel free to send me any path data / svg file with paths that behave odd.
I added a svg file with comparisons between the new and old behaviour
and proposal for "correct" behaviour which I might be able to implement.