Hello. I extensively analyzed how certain flags were set up, and I can say with certainty that the completion switch for the 2nd Basalt quest (#4819) is not tied to any other event, making it technically optional. Only the 1st Basalt quest is necessary for a certain FEX-related side event to appear, and I do believe this is indicated in the guide.
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Sorry, I should have clarified. You are right in saying that Basalt 2 is easy to miss, but it is an optional quest, meaning missing it doesn’t affect the true ending or FEX. This is why the guide only mentions the quest’s existence, but doesn’t include it in the list of ‘requirements’ for anything, because it is inconsequential. The Relic Lab event only requires Basalt 1.