The 8375 comes in lots of different flavours.
Yours is a 318069-17 which is a 1MB part, NTSC only. The PAL/NTSC pin has been repurposed to Vbb.
Your board appears to be correctly configured for this revision of the Agnus.
http://www.amigawiki.org/lib/exe/fetch.php?media=de:parts:agnus_reworks.pdfAnd as you suspected, it is not possible to force to PAL in hardware, you must do it in software. If you wish to convert this machine, you should find a 318069-16 chip, which will be directly compatible with your motherboard configuration. I also recommend swapping the 28MHz oscillator to the PAL variant for full timing compatibility.
