Of course you can, people describe other people as "being a literal donut", so I think "inane" is quite fitting when describing someone.

Yes, you absolutely can... People may get offended though.

describe someone AS inane. Yes, you can. In the US also. Not sure what Al is talking about.

American English - i have never heard this before so you won't be understood in the USA. If you have an accent i guess people will think you don't speak English. I know that's harsh, but just giving you my opinion. Anyway, maybe in Britain or Australia, it makes more sense - i don't know. :-)

Yes It’s usually used to describe something about a person, rather than the person itself, but it’s perfectly ok. Often we say a person has inane behaviour, habits etc