[Woodstock]

I would not subject my corals or fish to cooler water than what they're used to. Stability is the key to success and while using cooler water will probably not kill them, it certainly does stress them. If you had more sensitive corals like some sps, exposing them to cooler water would make them bleach(IME); a clear indication that it is not healthy for them.

Why not just add a power head/pump and heater to the mixing container? I use a 20 gal Brute trashcan for mixing new salt water with a pump and heater. I turn off all pumps, remove 20 gal from themain display (sometimes I vacuum the overflow and sump too); then I place a hose on the pump in the Brute container and pump the new saltwater into the main display.

Using cold/room temp top off is not an issue.

You should be able to turn off all power to the tank without causing a flood. The sump's water level should be low enough to allow plenty of room for the 'back flow' of water into it which occurs during power outages (or when you turn off the return pump(s)).