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.
It depends. If the portrayal of the yandere girlfriend is done in an interesting and nuanced way, it could be appealing. But if it's overly stereotypical or one-dimensional, then no.
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.