Can you be good at everything?

Definitely. I really value my free time - I would much rather work less if I could get away with it and spend more time improving myself.

In academic subjects, often that's the case. With more vocational/creative subjects like Art, Music and Drama, only to an extent. In all three cases, you can have someone who works like mad and really believes in themselves but has little natural talent, and you can have someone who does no work and thinks they suck but are actually naturally talented, and the latter will 99% of the time do better. Certain amount you can learn, the rest is just in your genes. Plus, you have to have an interest. I will never become an amazing physicist because I despise mechanical concepts so will never have the motivation to work at it.

There's no reason why not, so it's probably possible.

Nope, it's impossible. Just take one subject, for example, law. You can never learn everything to do with law - you'd just be crap at it. That's why there are so many "general solicitors" e.g. wills, marriages, etc, which require mainly procedural knowledge, then you've got the extremely specialist tax barristers - who spend their entire life on just one area of law, because that's what they need to do to be "good" at it. Then there's everyone in between.

Same principles apply to all other subjects. You can't be a nuclear physicists and a theoretical physicist - you'll be stretching your knowledge too thin to be "good" for those areas. You can't be a vet and a doctor. You can't be a knee surgeon and congenital heart surgeon.

You can certainly be "mediocre" in many things, just not "good" in everything.