German Brewing

If you rise the malt pipe to a level where no oxygen is sucked under, the wort will drain from the pipe without oxygen getting in. It will actually be kind of sucked down.
Working on new methods and tweaks every brew but I brew to seldom to have been able to reach any conclusion yet.
Overflow and no sparge (most of the times). Floating mashcap from a plastic fermenter bucket (perfect fit).

It may not be optimal, but it may not do as much damage as you think. It takes quite a bit of time and diffusion in order to mix oxygen into wort. You may only damage a negligible part slightly. This may not be enough to be perceptible.

The fact that you've been very happy with your results speaks for itself.

Otherwise, when in doubt: measure.

I have been brewing with the 10L malt pipe in the 20L BM. I don't seem to have that suction effect there. Question: in order to have the wort level above the 10L pipe I need to add around 19L of water. What would be the typical amount needed for a brew with the 20L pipe?

You need 36 litres to completely cover the bar and locking nut (that's on the assumption that you replace the wingnut with an ordinary flat nut). If you're just looking to cover the maltpipe, you'll need 31 litres.

Ok, that's simply double. But unfortunately the final volume won't fit in a keg.