That is not really how you do things. You would upgrade there is no option in the app to do what you want.
Your style issue you might want to look at your header as your calling another stylesheet in addition to the style we call so your custom stylesheet style2.css is probally causing your issue.. Remove that style from your header does your gallery tables look correct now?
I had this when I first upgraded, and it was because I was using an old stylesheet from v7.1. Have you removed all the old stylesheets and only have the v8.0 ones in the folder. I removed all the old files and folders on the server that are not part of this new build as some cause this sort of conflict.
Are you making sure you are using out latest gold release and not an old beta release?
I would suspect you are not using the final release. I also see you have modified things some. You might want to backup any changes you have done to templating and then properly update all your files to the current release replacing all files and templates except for the config files as the upgrade text states in the documentation folder of the download. You can then test out the code and if everything works then you can reapply any mods after that.
well your issue is the layout you have. Your using the new layout but your turning off the sidebar globally so your categories box is not being seen.
We make an assumption here in the code that if your using the new layout your using the sidebar throughout the site not just the index so you will notice if you turn the siderbar on globally your child categories are indeed shown. If you want to keep your sidebar off globally then maybe an edit would do. Maybe in showgallery.php this code add what is in bold.