I see - in that case this example should cover what you need (but I’d recommend setting your output function (in step 1) to Disabled (0), since you’re not using a predefined output function) 
Indeed - by default none of the data lake variables are directly used as inputs to commands that get sent to the autopilot, and especially not the MAVLink ones that are populated using values received from the autopilot. You can, however, use them in and/or to trigger Custom Actions if you want to