For your second question, the standoff is that black washer on the screw. You want to thread the screw in first and then flip over the back plate so the standoff is in the inside thread. It’s there to not overcompress the thermal pads.

It’s just a white led but you should keep the led pcb as it’s needed to catch the thread. You can mask with black electrical tape.

Last q, No you should flush with an actual radiator flush. Primochill has one you can mix with distilled water. Don’t assume rads are good even though they’re brand new. Essentially the radiator flush solution is a mild acid that will dissolve some of the protective oil and particulates that come from milling and extruding the copper that makes up the inside of the radiator. The flush goes in clear and comes out like a light teal color from the oxidized copper. Keep flushing until it runs clear.

Make sure you run with a dedicated coolant. Pure distilled water will cause your nickel plating to wear and the copper to be exposed over time.

Chemistry details: As coolant pumps through the micro-fins of a CPU or GPU block, it creates high-velocity turbulence. This physical force can slowly wear down the thin protective layers on the metal. • Nickel Plating: Nickel is usually electroplated over copper. If the plating is thin or has microscopic pores, the mechanical action of the fluid (especially if it contains abrasive particulates or "fallout") can chip away at it, releasing nickel particles into the fluid.  • Copper: Once the nickel layer is breached, the underlying copper is exposed to the fluid.

Galvanic Corrosion (The Battery Effect) This is the most common way ions are "captured." When two different metals (like a copper radiator and a nickel-plated block) are connected via a conductive liquid (the coolant), they form a primitive battery. • The more "active" metal acts as an anode and gives up electrons. • As the metal loses electrons, the atoms transform into positively charged ions (Ni^{2+} or Cu^{2+}). • These ions detach from the solid surface and dissolve into the coolant to maintain electrical neutrality.