Not sure I understand the problem. What condition do you check to trigger the event?

Game.creeps.creepName will become undefined when the creep is dead.

Game.getObjectById(creepId) will return null when the creep is dead.

But Memory.creeps.creepName persists even after the death of the creep.

Can we have Room.powerSpawn and Room.observer properties to simplify an access to them? Similar to the Room.storage and Room.conroller that are also only one per room.

Yes.

Some players avoid those resources, others do source keeper mining, even using different tactics. With the recent changes it became almost useless to mine those extra resources. However it's still an interesting task to implement.

Can the Source Keepers spawners be destroyed?

No.

TOUGH does not generate fatigue

I believe it's not correct. Just tested it. Created a creep with [MOVE, TOUGH, TOUGH, TOUGH] body and it moves every 3 ticks.

creep.say() method takes 0.2 CPU while should take none according to the docs.

var t0 = Game.getUsedCpu();
creep.say("something");
console.log(creep.name + ' ' + (Game.getUsedCpu() - t0));

I think we should protect the wall and not let the top players expand into the novice sector and grab our resources