I would ignore them. I think the meaning changes based on the context and how one uses it. Like how "bat" can mean an animal, sports equipment, or the action of hitting a ball when used in the phrase "up to bat". In the case of the song, the meaning of eldest daughter will depend on the lyrics

It can technically mean both, but usually just the oldest out of just the daughters, so you are one. Taylor likely just means it as being firstborn who happens to be a girl if she's using it to refer to herself.

Absolutely! They usually do better in the second half anyway

Well....