In the engine compartment fuse box, the orangish yellow 20amp fuse second from the right is the culprit of my electric drain.

It is labeled 'side light' or just 'light' from the Discovery manual.

Does it control any important system and is it safe to disconnect it?

 

You could remove it but you will loose some lights. My concern is that you have an electrical draw, how do you know it is the relay and not a bad wire harness? You really need to track down the true issue and fix that.

 

Like maybe corrosion on one of the affected light fixture could be responsible for the higher current draw?

Or a possible short from power to ground somewhere along one of t hose lines?

 

disconnect the light switch to see if it goes away. if it does its the switch and beyond. If it doesn't its from the fuse to the switch. to eliminate the switch put a jumper on the connector on c201 pin 3 brown/green wire and then jump the circuit to the switch pin 3. then ground pin four at the switch to see if it goes away. if it does go away its past that. if it doesnt then its the switch. Chances are its the switch itself. the switch itself needs to be open to ground the power.