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When I was at Marty Lanham’s shop in Nashville we talked about humidity and his number was 48% relative humidity. I go with that. Guitars will tolerate more or less for periods of time but if humidity is below or above the ideal, the further away from the ideal you go the shorter period of time you have before it begins effecting your guitar. You could have your guitar in the 20s for a day or two or the 70s but if it stays there you will risk cracks or buckling (popping braces and seams) The longer it is in the extremes the higher the risk of developing problems. I know a guy in Burlingame CA with a shop full of high end vintage guitars and he does not control humidity. There are a few days where humidity will run in the mid 30 but it gets followed by a few days where it will be in the 60s with most days around the high 40s. The averages equal out as long as the period of exposure is not prolonged as in Weeks to Months.
I would open that guitar up and play it as soon as I got it and not even worry. If you want you could crack the case for a few hours but as long as it was not subjected to extremes you will be okay. I would want to inspect it right away any how to make sure it was as promised. That includes playing.
I love vintage and most of the vintage stuff (exluding strictly collector,not player guitars and don’t even get me started on that weird genre) have cracks. It is not a big deal. Cracks are easily fixed if dealt with in a timely manner by skilled hands. Same with brace and seam issues. Many fixes cannot even be seen. Not the end of the world for either but rather part of how guitars are.
Play play play that new guitar and Happiest NGD to you!!!