1. Advantage does stack. Ludicrously, somebody quoted the specific rule explaining how they stack while simultaneously claiming that they don't stack. That is either deliberate absurdity or the very definition of a bad faith argument.
I'm not sure who you are referring to ('someone'?), but if that was me, I was simply pointing out that advantage and disadvantage do not combine to give your either - if you have both on a roll, then they do not 'stack'. Your roll has neither - so it's a regular die roll. The rules state that, very clearly, in the section I quoted. I thought that it was being claimed if you 'add up' all the advantage (+) and disadvantage (-) sources, and you end up with more advantage sources (so net +), then you have advantage. That is wrong according to the rules.