Jump Plus manga encompasses a wide spectrum. Whether a manga is seinen within Jump Plus depends on its specific content and intended readership. There's no strict rule that everything on Jump Plus is of the seinen category.
Sure, Manga Plus by Shueisha is legal. It operates within the framework of copyright laws and partnerships with the creators and publishers to bring manga to fans in a lawful manner.
Sure, Manga Plus is a legitimate option. It provides a legitimate and quality manga reading experience. You don't have to worry about any illegal or unethical aspects when using it.