While there is truth to everything you say, I question the importance of horses in the Andes. Horses are not really at all useful at the kinds of altitutes where the Incas dwelled.
The key to the Spanish conquest was that the Inca empire was full of internal conflict, the most important of which was the lethal enmity between the sons of Huayna Capac. The Spanish never really had trouble finding native allies, who were only too happy to get some rival tribe to be 'wacked' by association with the conquistadores.
You're right, native allies were important for the eventual conquest of the Incas. I was referencing the key Battle of Cajamarca where ~160 Spaniards slaughtered thousands of Incas (https://en.wikipedia.org/wiki/Battle_of_Cajamarca) and took Atahualpa, the Inca emperor, hostage.
The key to the Spanish conquest was that the Inca empire was full of internal conflict, the most important of which was the lethal enmity between the sons of Huayna Capac. The Spanish never really had trouble finding native allies, who were only too happy to get some rival tribe to be 'wacked' by association with the conquistadores.