The former is definitely interesting, even if it would have to come from the Galapagos Islands since Rapa Nui is fairly far south (and remote from South America). They'd be a great addition to coastal South America's crops. A part of me thinks that it could result in a Chimu wank, since they were the most powerful coastal state in South America at the time the Polynesians were there. Although I recall another TL here (Land of Sweetness) which also had a heavy focus on indigenous trade where it was actually the Mantenos who get that boost and create a large empire in the Ecuador-Colombia-northern Peru area. But I like the Chimu because they seem like an interesting counterpart to the Inca--the Inca's foremost god was the sun, the Chimu's was the moon, and the Chimu were a coastal state based on trade (IIRC) while the Inca were a highland state and a strict command economy.
I think for the coast of northwestern South America, the ideal would be to reduce the highlands into a zone of exploitation. They send valuable goods--metals, llamas, wood/charcoal--in exchange for shells, local manufactures, and trade goods (possibly from as far as Mesoamerica so things like chocolate, incense, etc.). This would also break up the command economy that was traditional in the region and make it more like a typical corvee system as found all over the world. Basically a "monetisation" of the economy (although I'm not sure how complex the shells = money thing was compared to how economies like 13th century Japan were re-monetised because of foreign trade and a newfound proliferation of coins).
Mulberry paper I'm not sure. They could've just used mud bricks as tablets if they evolved an alphabet, or perhaps have evolved their own script if necessary. The highland Andeans used quipu--woven knots--to record certain information even if it was more of a form of accounting. There was indigenous Mesoamerican "paper", so I'm sure there's some native plant in that area would could have been made into paper if necessary.