The other point is that the conquistadores, even when originally spanish, they ended aso being part of Latin american heritage... Im latin american and I lived in Spain for a while. Most Spanish peoplle don't even know much of how the Conquista was. So i think the right move was to base the adaptation from the input from latin america. (It´s like you were going to ask to the british about how would you portray the pilgrims that came to north america when adapting it. They descendants, by blood and cultural heritage, are not there anymore)
...Well pirates make sense in Ixalan when you take into account that there were British privateers and french corsairs attacking the Spanish colonies in America during the colony years, so their presence is not a mish mash, but takes into account the source material.
Also, some of the Legion of dusk art mimic the spanish baroque art, like Velasquez, or de la Ribera, which is a nice touch
(i had made a longer post with references but got flagged as spam lol)
On the issue as pirates "not fitting" into the set, I must disagree: During the spanish colonies in Latin America (for example, in my country, Chile), the colonial ports were continuously under the threat of the attack of corsairs sponsored mainly by the british empire... aka pirates (as in Pirates of the Caribbean)... so it makes sense that the mesoamerican setting to have the declining native empire (maya), the rising empire (aztec), the zealous/religious invaders who drink blood (Spanish conquistadors) and the British and french Pirates... they may have not coexisted in the exact same period, but they make sense together if you taake into account the real world inspiration.
Spectacle X (If this is the only card on your hand, you may cast it by paying it's Spectacle cost)
Also, some of the Legion of dusk art mimic the spanish baroque art, like Velasquez, or de la Ribera, which is a nice touch
(i had made a longer post with references but got flagged as spam lol)
I also like how some of the Legion of Dusk (http://www.mtgsalvation.com/cards/ixalan/29241-deathless-ancient) arts reminds me of the Spanish art of the renaissance and Siglo de Oro (like Ell greco, Jose de ribera, Velasquez)
More references on pirates attacking spanish colonies:
https://en.wikipedia.org/wiki/Buccaneer
https://en.wikipedia.org/wiki/Jean-François_Roberval
https://en.wikipedia.org/wiki/Francis_Drake