The hanson bolt extractors look pretty good. I have used the kind that look like a socket, but are really tight, and you pound them on, then take the bolt out.
If I come across a bolt that is rounded over, I have used the dremel method described, which worked pretty good. My friends honda timing belt tentioning nut was stripped so that is what I did for a temp fix until he got a new one. And I also have a full set of easy outs, and I love them because the left handed drill usually gets the bolt out already. And when you drill through its goign to drain the oil, then you can use the easy out to get the plug out.
I would invest in the mercedes tool for loosening the oil plug. Try this number 000 589 24 07 00 or 117 589 02 07 00. You can also use it to service the differential.
You can get them at the mercedes dealer or http://www.samstagsales.com/mercedes.htm
my mecahnic actually got mine for me, but I know that he got it for me from the dealer because its actually not too expensive. Since I got it, I havent rounded off any, but I replaced all the drain plugs on my benzes since I got it to ensure that I dont mess any of mine up. Make sure not to over tighten them, and always use a new crush washer.
I dont know if I would trust a drain valve because mercedes engines are really expensive....they hold pressure til the last quart, then boom they are dead. But for the ML270, that seems like the only good option...Getting oil everywhere is bad.
Let us know how you eventually get it off,