Can do this in my Child-Theme Folder? Or have i customize it manually in the Plugin-Folder?

You should make changes to the wpDiscuz plugin files.

I'm really sorry, but there is no way to make the customization in an update-safe way. All changes, you'll do will be lost after the plugin update.

All changes will be lost after the plugin update. You should keep the changes somewhere locally to re-implement the changes after the wpDiscuz plugin update

I want to split the "Subscribe to Comments" and the sort Buttons for better Usability. I want to integrate that Subscribe-Bar right beneath the Comment-Form and show the Sort-Button in the Top-Right Corner of the Comment-List. For that i want to customize the template File comment-form.php. 

Can do this in my Child-Theme Folder? Or have i customize it manually in the Plugin-Folder?

Optional: Maybe you can solve my "problem" a better way? For better understanding, i'll attach a Screenshot of my Imagination.

http://prntscr.com/pouyo7