The “long” product image is the size of the image that you use. The theme cannot change this, sorry.
When the mobile menu is opened, the page is moved out of the way horizontally. This is the design of the theme, and it is not an error. We think it looks better than having the menu overlayed on top of the page content, as the mobile menu slides in from the side.
If you don’t want the page to move when opening the mobile menu, add this CSS to ‘Customize‘ > ‘Additional CSS‘: