a) To remove the sidebar on Nishita, you simply go to Appearance > Widgets and make sure there are no widgets in Sidebar (delete them or move them to the Footer areas).
b) The Custom Design upgrade allows you to change the default styling of any element (fonts, colors, whatever). Without the upgrade you can change the font color using code in the post/page editor. I can show you how to do this, but I would advise against it: Nishita's grey is fine, pure white on black is too blatant.
c) The scaling tool is a crude image editor, to be used only if you don't have an image editing application. And it's designed for scaling down only. Also, it's used before inserting an image in a post. What you see on a post depends on its code, and the code doesn't change if you change something in the media library after you have created the post. In other words, forget that tool: to correct images inserted in posts/pages, you need to edit the posts/pages, not the images.
Correcting individually inserted images or galleries is relatively easy, and I can tell you how after you link to specific examples. The WP slideshow is a different story: the max displayed width is fixed for a given theme - you can't change it.
By the way, uploading 2426px or, worse, 4180px wide images is simply a waste of storage space. More on this after you tell me if you want or you don't want individually inserted images to link to the full-size original alone when clicked.