1. Actually i’m not really know much about NextGen Gallery plugin. Yes, for fullwidth size post will use 1150px width container. In this case, since you’re using NextGen Gallery, you may need to check your Gallery Settings for NextGen Gallery. If you’re using Add Media to insert an image into your post, you can choose Full Size on Size option or Custom Size.
2. & 3. At the moment i can’t check your site because it doesn’t load properly on my side. Can you tell me what version of JNews that you use on your site? We just released a new version of JNews today v1.1.2 including several plugin updates. Kindly you can check it out.
5. You can find out this file /jnews/assets/css/main.css . But if you don’t get the same CSS code on that file, maybe they come from a plugin. You can put your changes on child theme or Additional CSS, choose which easier for you.