To open a manga cafe, you have to have a good collection of manga. Also, create a cozy and inviting atmosphere. Offer comfortable seating and maybe some snacks and drinks. And don't forget to market your cafe well through social media and local promotions.
Having hentai in manga cafes is a big no-no. It's not the kind of thing that's acceptable in such a setting. Manga cafes are for general reading and entertainment, not for adult-oriented content like hentai.
Definitely not. Having sex at a manga cafe is inappropriate and against the rules and purpose of such establishments. It's a place for enjoying manga and having a quiet time.