Reconsider the 15 minutes you spend in line at the café to get coffee．That's a quarter-hour more you could be sleeping if you bought a coffee maker with a timer．

问： That's a quarter-hour more you could be sleeping ，改为You could sleep a quarter-hour more .(你可以多睡15分钟。)而不可改为：You could sleep for a quarter-hour more .(你可以睡觉持续15分钟多。)对吗? 

问1：那为什么用了进行时态而不用 could sleep?

问2：you could be sleeping是定语从句的话，为什么它的引导词可以省略以及它的先行词是什么？