Looks like a magnesium def to me! Certainly not n.p.k, add a few doses of Epsom salt, dissolve it in hot water and add it to your feeding water! 1/2 teaspoon per litre! :)
Yeah man they start yellowing at the end of flower but that’s tip burn from nutes at the end of your leaves, not the end of the world its nearly done anyways :)
Hard to tell really dude, it may take more than 3 days from 12/12 to find out, do you have other plants around it in the same room? Or on its own? Looks like sacks are forming but not 100%
Yeah man best with a dehumidifier, opening windows is good but is never really an option if in a closet or tent, it’s harder to bring down humidity than it is to raise it without electrical help! it’s worth the investment if you want to reap the rewards, just like borrow one of your mum or uncle...