How does double XP work

It's altitude based, not terrain based. Just because you're skimming low over a mountain cliff doesn't necessarily mean you're under the radar barrier altitude. Keep the map's geography in mind. When you want to be below radar you must be near sea level.

You will get 20 additional spot points if that enemy is killed after you spot them with the MAV (or T-UGS). And potentially a ribbon if that happens a few times in a row.

Also, you can left click to lock on and blow up enemy equipment, things like the recon mobile deployment beacon and other MAVs.

It's worth noting that the MAV also includes a passive detection ability exactly like the T-UGS -- if you leave it in a location sitting by itself, it will send out periodic "pings" that highlight any moving enemies nearby. If you don't want to be tied down piloting the MAV, it's a good strategy to drop the MAV in some out of the way area where it won't be spotted easily by enemies, and let it passively detect enemies even without you operating it. It is more effective when piloted, of course, but it also works with no human intervention too, kind of like SOFLAM.

Note that as of the latest patch, the MAV will destroy itself shortly after you respawn. It lives only as long as you do, including the time you are dead and waiting for respawn, it is not a permanent artifact once deployed.