Hey Bro! wow i see you've put alot of effort in getting this tank new again! Well done and i notice you removed the RHS door.
As for the overflow problem the sound will never stop even with a ball valve. You need a durso pipe, reason being when your tank water evaporates the water in the overflow compartment will drop and you'll have the sound again. I tried that before, so my advice is to use a durso pipe.
The thing about our local pipe dimensions is that an 3/4" elbow will not fit into a 3/4" T-joint. So what we do is to cut a 1" pipe to connect the elbow to the T-joint.
There is an alternative way which i will post when i home which i done w/o using the above method.
As for your skimmate don't have to worry much as for now you are cycling your tank and not much skim to be skimmed out. Once your cycle is over and you've added in fishes, skim muck will appaear.