You realize localized weather is not always predictable far enough in advance to do much? Moreover, airlines don't require passengers specify their weight when they purchase a ticket, so they can't really plan ahead for going over a specific weight that is itself tied to local weather conditions. Mind you, this could be avoided by building in more wiggle-room, but that is not going to be accepted as a solution because it results in waste much of the time if, for example, you have empty seats because you wanted to be sure that you wouldn't run in to the issue of going over weight.

Yes, less air means less lift. Same issue with altitude.

Well that would be the logical thing to do.

Kick of the heavier passengers and they don't have to remove as many total people. Which is overall better as you're creating travel problems for fewer people.

Although unless you're like 20st then it's unlikely that it would matter a great deal whether you pick the weightier people since they'll be making the calculation based on a formula that attributes a set value to passenger weight.