BestWebSoft Gallery Plugin customization

I have this gallery plugin bestwebsoft.com/plugin/gallery-plugin/ installed and I’ve been trying to restyle the gallery to be more like #3 mixed with #6 of these examples https://juverman.narod.ru/slideGallery/Demo/index.html but I’ve tried different scripts and doing it just sort of via css + onClick and swap functions for the images (very simple, all in the thumbnail img code so that clicking a thumbnail will display its full-size image in the place of the full-size image to the left of the thumbnails with the id/name “cover” which is its target) — except I couldn’t get that to work either, but I had it working on my non-WP draft. I think the trouble might be in linking the custom CSS and more importantly JS files because the css issue I remedied by just pasting the styles into the Gallery Plugin stylesheet. but the scripts themselves I can’t get to work, and I can’t seem to figure out how to have the full-size *featured* image for the gallery display by default. Right now I think it displays the first one by default instead (full-size).

Any help is much appreciated, thanks a lot.