To conclude the saga -- I blitzed all Python & XCode from the machine then reinstalled Python 3.10.6 via Homebrew, including Command-Line Tools 14.0, downloaded latest OCLP 0.5.0 sourcecode, ran
pip3.10 install -r requirements.txt
to install a heap of dependencies, then the build worked very smoothly and resulting GUI.app suffers no more nasty glitches ... YESS!!
Built an EFI spoofed as MBP14,1, copied to USB key & booted with it into external Ventura_B8_SSD, then root-patched that with Haswell5000 Graphics, which after a reboot gives full acceleration and is looking very presentable indeed.
View attachment 2082787
The GeekBench 5.4.5 GPU Metal test still crashes the machine on the Canny task (as in Monterey), but otherwise the results are excellent, all keyboard controls, trackpad gestures and fan speed regulation work perfectly and the machine seems very stable on Ventura.
Am delighted with the results and foresee making Ventura my daily driver on this machine once 13.0 final is released.
Thanks again to all who assisted above & OCLP-Team for getting it this far!