Hi,
Is there any way to hardwire the amiga to NTSC?
Yes, I dont know what it is on AGA Amiga`s but on fat Angus Amiga 2000`s and 500`s there are two pins, one is PAL and one is NTSC, I think the one pin is earthed (Or pulled up) to force it into that mode.
Dont know about the AGA chipset though....
As for the composit thing, I have the same the other way around (NTSC system in the UK), It must be a timming problem (Change in crystals), There is something like .03Mhz difference between an NTSC and PAL Amiga, Again on the OCS/ECS chipset you chanced the crystal to correct this but I dont know about AGA.
So there you go, Hope that helped you, Probably didnt though :-D