PF: If a candidate is registered in country A (for more than 12 months prior to the call deadline), but actually resides full-time in country B while conducting their main activity in country C, would they be eligible for an MSCA PF in country A?

Yes, the researcher would be eligible to apply with an institution located in country A.

The researcher's eligibility is determined based on their main activity and not solely on their legal or actual residence.