What can a Reaper drone do? Do they think if an unmanned drone does the job, China can't legally retaliate because an actual American wasn't piloting the aircraft? Is a Reaper drone like how they made a big deal about Vietnam obtaining six Flankers from Russia as if that meant Vietnam would have air superiority over China? Why Reaper drones when the they can just launch a full naval assault. They think one or a swarm of Reaper drone are going to destroy those islands? Oh yeah maybe their plan will work if they think they can tell China they can't shoot them down...
The use of drones is for them to be shot down.
If the US launched an attack from a naval fleet, said fleet become legitimate and immediate retaliatory targets = massive US loss of life and inevitable escalation to full blown war.
With a drone strike, the Americans attack some unmanned part of the islands to cause a lot of smoke for TV cameras but no loss of life. China immediately shoots down attacking drones in retaliation.
There is scope and draw a line there and then where both sides can claim victory and score enough political points to avoid further retaliatory strikes and uncontrollable escalation.
Trump gets to say he dropped the hammer on China and struck Chinese territory, that no other US president has dared to do since the foundation of the PRC. China will declare an effective no go zone for US military ships and planes within XXXX miles of Chinese sovereign territory, which will now include the SCS islands, with a shoot on sight policy for US military forces spotted in that no-go zone.
Trump gets re-elected and China pushes US military power out of its front yard.