OK, looked in the .ini yet again and temp was set for J. So made sure the directory existed (since I deleted it last night!) exactly as named in the ini which was simply J:/Sigil Temp ... and it still would not work. Got the same error. So reinstalled (without wiping the native temp folder in order to save preferences) and it opened up fine. Went to preferences and clicked Auto to put the temp directory back to the native location. Restarted Sigil.
And now we'll continue to see if the crashing has disappeared or not... will report back either way, but will be back sooner if it crashes.
Thanks again for everyone's help and time.