What are the odds of a rival European power responding to the Spanish conquest of Hispaniola, Cuba and Mexico by undertaking a competing conquest of Peru before Pizarro's OTL conquest?
The Spanish undertook these conquests with a tiny number of men, suggesting to me that really any Old World power with ships could have done it.
I'm thinking France, England, Scotland, Venice, the Scandinavian powers, the Hanseatic League or even the Ottomans (if allied with Morocco) as possible contenders, given that Portugal was focused on the Eastward route to Asia...