Apologies for something I should have been able to figure out.
Good news. Sigil is built. Tested a few things including converting to EPUB 3. So far so good.
Earlier, Sigil would crash on start without any log. From a shell, running Sigil.app/Contents/MacOS/Sigil listed some message about not finding the python encoding library. Maybe I missed something still despite going over it several times. Also found a few other refs to Python 3.8 in make files. Within Python.framework, there was no Versions/3.9, only Versions/Python.framework and Versions/Current. Adding a symbolic link fixed that.
I ended grabbing the MacPorts QtWebEngine 5.15.7 patches for Monterey mentioned earlier:
https://github.com/macports/macports-ports/pull/12595
That compiled fine. QtWebEngine is now at 5.15.8 though I didn't test it.
One additional patch was to remove the hardcoded x86_64 build arch from the qmake conf files. There's two listed from MacPorts, for pre- or post-install:
https://github.com/macports/macports...aqua/qt5/files
Trying to recall prerequisites that didn't compile for arm64, I think it was libpng rather than the xz mentioned earlier. Installed thru homebrew. Besides that, I think all patches are listed above.
Quite a bit of trial and error, particularly since I was unfamiliar with the various tools and haven't compiled anything myself in sometime. Yet now all is good and I can return to reading, making, and editing ebooks. Many thanks Kevin and DiapDealer for the assistance.