Displaying all galleries at once

Hi,

I am new to wordpress and need a few questions answered about using galleries to upload quite a number of images. The site I am helping with has 8 galleries and more than 20 images per gallery uploaded. I am using Colorlib’s Sparkling theme and it is brilliant so far. Because I am using galleries and not a page for each gallery I am having a few issues.
1. When the link ‘gallery’ is clicked on from the header and sidebar, I would like all the galleries displayed on one page with a thumbnail/hero image for each gallery. I am not sure how to do this. Would I need to download another plugin for this?
2. Because these are galleries and not pages they do not give me the option of a full width page. How can I acheive that within the Sparkling theme please?
3. While in 1 of the 8 galleries is it possible to have the before and next buttons above the posts rather than below linking it to the other 7 galleries?

Any help here would be greatly appreciated. Many thanks!