Interesting. That kind of blows my Windows update theory out of the water. The version of Python that comes with that version of Sigil (Python 3.7.2) is supposed to require the same hotfix that 3.8 does.
0.9.14 working for you would suggest that the issue is Qt/WebEngine related -- but then those python errors!... I'm afraid I'm a bit baffled.
If 0.9.14 works for your needs, great! But there were a lot of performance issues that people complained about with the version of QtWebKit that was shipping with it. If it proves to be too clunky and slow, you may want to try the official 0.9.10 release. Or better yet, my legacy 0.9.13 fork should provide all of the same features as the official 0.9.14, but should perform much better on aging OSes/hardware.
EDIT: it appears the original KB2533623 update (for Python 3.7/3.8 support) was superseded by the Windows 7
KB3063858 Security Update which is still available from Microsoft.