1. How can i setup that the Blogpage is in full width. (only the Blog page) It’s the blog page!!

The shared page is not blog page but single post page and you can make it full width by adding the following CSS code in the Custom CSS code option of your theme on the below path.

Admin Area -> Appearance -> Theme Settings -> Custom Code -> CSS Code

.single-post .post-content .product-content {
    margin-left: 0;
}

2. In the Theme Settings i can Setup a Blog Title. How i can change it to the Pagename. I tryed to create a Shortcode that shows “the_title();” but it doesnt work in the blog title. (On this Page you can see the Shortcode “[pagetitle]”)

You can try achieving this by using the attached custom child theme of Jevelin theme that contains some custom code.

3. You can see, i changed the Backgroundcolor to grey… now all links are having a grey background to. Why (Footer and header), how can i fix it?

You can resolve this issue by using following custom CSS code.

#page-container .menu-item a {
    background-color: transparent;
}