At what point does choosing an executor or other fiduciary stop being a family disagreement and become a legal dispute?

A good fiduciary candidate is typically reliable, organized, available, willing to take the role, and capable of managing conflict. There is no single national rule for probate, trust administration, title, and family claims, so have a lawyer in the state confirm the governing documents and the present court or title status before anyone does something that cannot easily be undone. Make a complete file of deeds, financial statements, beneficiary documents, court filings, and family communications before the disagreement becomes harder to reconstruct.