Actually Post #3 is so out of date we should delete it. The wrapper.py version to Sigil version can actually be found by using our github site tags much more easily.
As for Post #4, nothing much has changed since the Sigil 1.0 timeframe in the plugin frameworks. That plugin dev manual is still valid. The plugin interface has been and continues to be backwards compatible. There are a few recent additions (I count 6 addition since 2019) but the fundamentals are unchanged. Even the current guide (14) does not include really recent changes. As with any dev project, the best way is to learn it is to read the source and see examples and we have quite a large number of python plugins for people to learn from.
Update:
Actually, the rev 14 version of the manual is up to date except for the latest commit that was introduced in Sigil 2.2 that allowed the plugin to see what if any font mangling algorithm was set to be used by the plugin.
See:
https://github.com/Sigil-Ebook/Sigil...2bf385c18f3ebb
I will further update it to rev 15 with that one missing interface call, and then remove all of the references to python 2.7 that still exist and maybe make it clearer that python 3.4 in the plugin.xml file only specifies the very minimum python version.
Update 2:
-----------
Added an out of date notice on Post #3 with a pointer to using github on the wrapper.py file for any specific Sigil release tag they might be interested in.
Added Sigil Plugin Framework at rev15 (consistent with current Sigil) and added it to Post #4.