If what you want is not see the image but have the text be full width, you should proceed differently because as long as you are using position absolute things will be a bit funky.
What you want on mobile, is have a transparent color on content, so the background of the text is made by the background of the container, and so you need to set the container background color on mobile to be just that.
Use
#container {position:relative;background-image:url(ImageUrl);background-size:cover;background-repeat:no-repeat;background-position:right;padding:0px;}
+ .is-desktop.page-id-262 #content {width:50%;position:absolute;height:100%;overflow:auto;}
+ .is-mobile.page-id-262 #container {background-image:none;background-color:grey;}
+ .is-mobile.page-id-262 #content {background-color:transparent;}