Guy, at various stages it seems. At first it would crash when trying to navigate (final step) to the stored .rbz through the new extension manager. After trying a few times and a day passing, I tried again and was successful, then SU2017 would crash when I arrange menu items, from floating to docking, SKALP was not the only extension this was happening to.
I noticed that SU2017 would also crash even if I just left it open in the background.
Eventually I doubled down and reinstalled SU2017 making sure I didn't skip installing it as administrator. Things didn't improve. I then updated the graphics driver (already had a very recent version, but none-the-less did so). Things didn't improve. After this I updated Windows (everything except Silverlight and Skype, it was a short list though) and this seems to have helped. SU2017 has not yet crashed since and I have been able to finally set it up the way I like it.
Thanks for the interest in trying to help figure things out, much appreciated! In the end, I don't think this was a SKALP problem, more like a conflict somewhere between the SU2017 installation and Windows 8.1 (SL).
When will you be releasing the updated manual for SKALP version 2? It seems like it has now moved out of beta (thanks for the new features).