If I posted a video and click then on "Or click here to REUSE your last uploaded video!" the video croping feature wont work. At least on mobile.

-> the needed script for the video crop feature will never load on this way, you need to go over "Upload new video" to have this functionality

Same if I click on "Upload new video" and then click on "Reuse selected file for new move" before the social media buttons above the footer are loaded, the video croping in the editor does also not work.

--> You need always to wait for the social media buttons to be loaded (so the site is fully loaded including the needed script for the croping feature)