Generally, yeah. It's kind of like how we refer to the human race as "mankind". It's also like Spanish! Masculine refers to both boys and girls, whereas feminine refers to only girls. I usually try not to do that. Idk why, I just refer to gays (chicos y chicas) as "LGBT" rather than simply "gay people". Simply a habitual thing, dunno how common it is.
Yes. Don't worry, I ask my daughter seemingly obvious các câu hỏi all the time. She's not my only authority of course, but it's helpful as she's usually around. Plus it gives her the opportunity to make fun of me.
Well, the term "gay" usually means any homosexual person, but lesbian is probably strictly female. So bạn can't call a gay man hoặc boy "lesbian" because it wouldn't make sense. But I usually would call a homosexual girl hoặc woman "lesbian", but it's up to you.