You're calculations miss the third aspect of GL. It provides an increasing amount of additional damage and an decrease to your global cooldown, which you mention, but the part you overlooked is that it also decreases your auto attack delay by 5% for every stack. This isn't flashy or impressive numerically until you add it up over the course of a fight. As an example, looking at some of my old parses from Omega, my auto attacks made up to 30% of my entire damage when compared to our dragoon or bard having only about 15% of their total damage from auto attacks, adding more frequent auto attacks to monk will make a difference. To that end, I would say it is always better to be in fists of wind if you unless you have fewer than three stacks of GL.
Yes. You can apply Riddle of Fire while in Fists of Wind without losing your stance or 4th stack of greased lightning.