Can you toggle between PAL and NTSC by pressing any key while in the boot menu? If not then you would seem to have a hardware issue preventing you from changing to PAL... but AFAIK all you need is the newer Agnus which you have.
I never had a lot of luck with the boot menu switcher. You might try something like Degrader, or 60hzemulator. I used 60hzemulator a lot, it had a hotkey you could press to switch to PAL at any time, and I think it installed itself to survive resets.