If you're worried about the T fitting breaking apart, just use it during the flush, and replace it with a coupler when you're done.

Reverse flushing works better if you have a heater valve that can shut off the heater core inlet. That way, the flush will force water through the cooling system in reverse, carrying out sediments that get lodged in the cooling system from the normal flow. It's true, ideally if you always used distilled water and the proper coolants, you shouldn't introduce contaminants from those sources. But the coolant breaks down leaving byproducts, and engine itself is slowly reacting with chemicals in the coolant leaving corrosion deposits.