I was able to drain the heater core by disconecting the hoses from the heater core to the intake manifold and hooking up a shop-vac and sucking the old coolant out. You might also want do the same from the two hoses that go to the windshield washer tank heater.

The same trick might work on the block from the water ports once you take the water pump off. At least you could suck out some, refill with water, suck again. Repeat until the water coming out is no longer green. Buy some 1/2" clear plastic tubing and push it into the water ports to extend the reach.

As others have mentioned, the two plugs are very difficult to remove. They have a 6MM allen head and may strip the head when you try to remove them.

The MB manual describes a flushing technique that requires two tools (a tee fitting and a special thermostat). I checked out the price from the dealer, about $100 for the two of them. Since I had the cylinder heads off I choose to suck it out with a shop-vac. If you think your system could use a cleaning, then a flush would be better. If you don't want to buy the tools can always take it to a shop that has them.