This is essentially contradictory, you can't have the diseases both not really spread but also spread and somehow grant immediate immunity and somehow populations recover immediately from a 50+% decline. The less diseases spread the more it means they would spread when direct contact with Spaniards is made which is not really better, in any case talking about recovery within mere decades is a really tired cliche that has no backing to it.
Not having a civil war doesn't really mean anything in this context, the Spaniards have so many local chess pieces to manuever against one another like they did in Mesoamerica and any individual conquistador army would be stronger vis a vis any given smaller kingdom than they were against the unified Incas.
The luck factor goes only so far when the Spaniards consistently took over many regions all different, at what point can we talk about trends and not luck? Also I repeatedly said my assumption was the Spanish were as successful as IOTL, obviously you can postulate whatever you want but that becomes a pointless comparison, if we want to isolate a specific factor of decentralization vs centralization which was my entire point we can'T start talking about a dozen other variables.
A conquest fails, others would come and each time the Spanish would be in a better position to invade as the local population decline sets in, as the Spanish properly take over Colombia and Ecuador and explore the region better.
We can make all assumptions we want and explore the worst or just worse case scenarios for the Spaniards but at some point we need to stand back and look at the bigger picture and general trends, it's only going to go quickly uphill for the Spaniards especially in a scenario where the locals are divided and aren't going to unite soon lest they give the Spaniards another way to play divide and conquer like they did with the Tlaxcalans and other Aztec enemies.
You are looking at the situation and only factoring in the advantages of a divided Andes without looking at the disadvantages, such as the fact that outside the few larger kingdoms the Spaniards would fight such small polities than their conquistador armies can just roll over their enemy armies without even needing diplomacy or subterfuge and the strategic depth of each individual polity would be small.
I mean after all the Incas expanded from their core territory around Cuzco from 1463 within 60 years(with most of said conquest being in just 30 years), why in the world would the Spanish find that much more difficulty in doing something similar given their repeated success elsewhere?
1) yes you can b with some Andean areas being hit early while some later depending on if it remained very fragmented or maybe a group like the chankas conquered some more
"The civil war does not mean anything to this context"
The Inca had parts where 90% of their population died due to the combination of the plague and the civil war of Atahualpa and Huáscar
Even if chimu is devasted and it will be it still could muster an army or have the Spanish killed if they tried anything before their people join them
Speaking of smaller kingdoms
Who is to say they would join them sure some might for political gain some others might not or some others play both sides or some might unite against them others might become to help then backstab them
2) the trend was the deposite how many things we atribute to guns germs and other things luck was a principal factor ok why Spain succeed so much Moctezuma could listen to cautemoc and have the Spanish killed
The smallpox plague kills the Inca and his designated heir
The Spanish just arriving literally days as the civil war of the Inca empire was coming to an end and Atahualpa just being in his festivity not brining arm warriors
These where very specific circumstances that are like I said borderline abs Try as we might these factors where central to the fall of both empires with out them their fall is delayed and changed
3) exploring worst case scenarios ? The fall of the Inca was one of the best case scenarios Atahualpa or in this case the chimu king killing him and his men is not a worse case scenario is a realistic one
also in terms of Colombia it might fall as fast as it did in the otl or maybe little later in terms of Ecuador
Depends there is no civil war with Atahualpa taking most of the northern troops to figth Huáscar so Ecuador would not be as defenseless
4) why did the Inca conquered the whole thing in 60 years also yes 60 not 30 unless you consider the distance of aprurimac to Cusco be part of simple the Inca had better roads the mita system among other things allowed them to call large number of troops
the Inca had been expansionist before Pachacutec and had layed down for him the groundwork on what to work with combined this that Pachacutec and his son and grandson very competent in both war and politics I don't like great man history but it's true in this case that he had all the tools and he knew how to use them
the Spanish don't have these advantages they don't have the Inca system they don't have the numbers and need to make an agricultural basis conquering chimu won't give them this .
they don't have many native allies to flock to them to get Rid of the oppressive Incas they would with chimu the Inca did this but that's the only record of that
So to summarize no strong native empire to kill and take over quickly
Higher possiblity of pizzaro being killed by the king of chimu or any other than a whole ransom scenario
No that many discruntal natives to join you
Give you supplies guide you etc .
Many divided native kingdoms that could or could not be your allies may or may not backstab you
No Inca road system and wearhouses to easily move and resupply and no real knowledge about the terrain and how it can be used against you (in the otl pizzaro had thousand of native allies who knew but he kept sending cavalry to their deaths at the hand of quiso using the terrain to his advantage)