What theme version are you running? Please make sure to have the latest one and all it’s included plugins updated. If the warning persists, add some wp credentials in a private reply. So we can check closely.
The text is an element. To edit it hover over it and click to open the element’s options. To delete it completely, click on the bin icon. See screenshot: https://ibb.co/6XQb2tS
I suggest you change the browser or check if any browser add-ons are interfering.
I tried again and had no problem adding a new gallery and adding more images to it. See screenshot: https://ibb.co/rFdqCW2
Are you referring as the sidebar, to the menu bar? Some sections in theme hold a limited amount of elements. This is the header section. Sorry but it’s not possible adding an image to the header.
Most probably the permalinks reset did not take place. Can you please try resetting the .htaccess file (find it in wordpress main folder, or ask your host)? Just rename the current one and reset permalinks once again. This will recreate the .htaccess file again.
I tested this on my installation and it works well.
I see some of the text is set up as a link, this disables the selection using the mouse. Other text is set as heading and has other classes wrapping it up. I suggest you open the page in the editor and copy the text you want from the text editor(screenshot). This is not really something we can do anything because it depends on how you have organized the content on the page.
Please clear cache and try again. I opened the site in both browser responsive view and actual Samsung S6 device. These are the screenshots: https://ibb.co/4mdHtpj https://ibb.co/6Dp20tD
