owkey, I've done it so I can anwer my own questions:
-Yes need to drain the coolant water via the radiator plug AND after that there is also a lot of water coming out the tube and waterpump.
-The tube itself came out easily, as expected, the o-ring on it was broken. Replaced it and added some minor RVT sealant what did the job.
But now I have a leak at the radiator drain plug
It's tightened by hand which is obviously not enough. Am I allowed to use some pliers for that ? Or is there an O-ring which needs replacement each time this tab is opened ?