I don't think PAL vs. NTSC matters too much for monochrome monitors.
I have a IIc and a Monitor IIc as well, plus the IIc PAL adapter. It doesn't matter whether I plug my (220v, European) Monitor IIc into the IIc's composite monitor port or into the PAL adapter's composite port.
I also have an original Raspberry Pi that I bought in the USA: it emits NTSC for sure. I plug it into the Monitor IIc and it works fine.
My conclusion from all of this is that the Monitor IIc is fine being given NTSC or PAL video.
ETA: There are several versions of the Monitor IIc with different electronics inside. I have the Samsung internals.