mct16:
The Incas certainly have the means. They have enough influence to put a whole ship under so-called quarantine and make a whole village, including the local police, keep quiet about their activities - "No se", "No se" - with only Zorrino defying them out of gratitude to Tintin.
The Incas have influence - but, it seems to me, only in certain places. The police chief from Callao appears to be completely ignorant of the Inca influence over the Quichua population - see P.5, f.7. He saw no significance in the fact that the doctor was a Quichua, despite being a top police officer in a large city. Had the doctor not been an Indian - and surely a lot of Peruvian doctors were not - the ship would not have been quarantined.
How did it happen that it just happened to be an Indian doctor on duty? My guess is that it was either co-incidence, or that this doctor volunteered. In the Peru of Prisoners of the Sun, it seems to me that people of Spanish descent are in all the top positions - and I would imagine that the Indians were in no position to dictate that a Spanish doctor checked out the ship.
mct16:
Even if a car was available in the area, Tintin and Haddock appear to be short of money. Tintin cannot afford a train ticket on his own and they've given up on assistance from the police. This was at a time before ATMs and getting a money transfer via a local bank may have been a tricky and lengthy process.
I put a different interpretation on the fact that Tintin could not afford a train ticket. In most Tintin books, money is no object, and he can afford anything. I therefore would assume that on this particular occasion, Tintin having gone out at night with the intention of swimming out to the
Pachacamac, only carried a small amount of money on his person - but that he and the Captain had their usual large supply (undoubtedly sufficient for not merely renting, but actually buying a car) somewhere else - like back in the hotel safe.
mct16:
Even if they did find a car in Santa Clara they'd probably be charged too much and Huascar may already have blocked all attempts to hire one - in a way similar to how he coerces the railway guard into committing sabotage.
Here, the question is the geographical. Callao (where the Pachacamac was docked) is a suburb of Lima. The population of the Lima/Callao area was over half a million in 1940, and would have been heavily Spanish, rather than Indian. When Tintin and the Captain meet up (page 12) they are clearly within walking distance of Lima/Callao (because they have walked there from the beach near the Pachacamac), and could surely have walked back there (to a Spanish city, where Inca influence was negligible, and cars relatively easy to obtain - rather than to Santa Clara) in order to get a car. The fact that they did not do so suggests to me that travelling by car to Jauga was not really a realistic option - or at least not if one was in a hurry.
Well - that's my thinking, anyway!