The only way to get the heavy stuff (rusty scale) out of the engine block is to remove the drain plug, but if you don't see rusty water coming out of the radiator when you drain it it probably isn't worth a block flush. And with that plug being stuck in most cases and very hard to remove without a long extension on a a 1/2 in breaker bar, and using heat, its not worth the effort - just use the chemical flush. Drain out thru radiator and fill thru the S shaped heater hose at the L side where it leaves the block and passes by the oil filter cannister. I've never thought of filling from the mono valve. You want to fill the head to prevent an air bubble, I don't know if the mono valve will pass fluid back to head without going thru the heter core or maybe it OK, I don't know, I just always fill as I was taught - thru the head.
I also stick a hose nozzle (the kind that has a tapered brass end, set to "stream", and run the water full blast into the head and let it come out the radiator, you can remove the lower radiator hose too to get it out faster.
After the chemical treatment flush the system with water at least two times until the discharge doesn't feel "slippery". I usually flush 3 or 4 times before I feel the discharge is clear.
Be sure to drive the car a mile or so to warm up the engine to operating temp and open the heater each time before each flush. Let the engine cool about a half hour before draining the system so you don't hit it with cold water while its hot. Properly done, this entire process will take around two to three hours.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!