Yes, it is generally legal to use public domain quotes in novels. Since the quotes are in the public domain, they are not protected by copyright. This means that authors can freely incorporate them into their works without having to seek permission or pay royalties.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
In most cases, you can. However, you need to check if the name is available and not protected by intellectual property rights. Also, consider the potential legal implications if the name is too similar to an established brand or entity.
Nope. Ryomen Sukuna has no basis in reality. It's a product of the creative minds of the writers and illustrators who designed a captivating fictional character for the enjoyment of the audience.