Nice video and neat setup. I think the issue the dealer is referring to when he tried to hook it up so stick-right would move it down and stick-left would move it up is the regen. function. You can do a search here on what regen is but it's common to employ it in the FEL dump/curl circuit. As I understand it, with regen, hydraulic oil moves from one end of the cylinder to the other as you dump the bucket thereby giving you a more controlled dump. If you push the stick all the way right past the detent (which you should be able to feel), you'll go past the regen feature of the FEL valve to fast dump, where the hydraulic oil is sent back to the tank. I don't believe that is the same as float, which you only have on the boom/up-down circuit of your FEL.
But, you could use the FEL boom circuit with the float feature to run the sickle-bar mower. Might be intuitive since pushing the stick forward would drop the mower and pulling it back would raise it like you're use to with the FEL. If you could mount some sort of anti-scalp wheel on the end of the mower and put it in float, it might follow the ground contour nicely, except for mole mounds or the like.
Let us know how it works out after you've used it some more.