Hi Valentine
I'll leave more experieced people to comment on many of these matters but with regard to where you put your tongue, just behind my tongue works for me and feels comfortable. I can't believe that up in the rounded bit wouldn't work but it feels less relaxed and I am beginning to feel that relaxed is right in this regard. We'll see what others say.
In Harmony of the universe I just tried to straighten my knee and it felt very odd. I've always bent the knee on the same side as the way I'm leaning and kept the other straight. I don't have the manual to hand - can you make it out in that or in the video?
Breathing in this way felt odd to me at first and it seemed as if i couldn't get a 'full' breath. Now it seems just fine to me and I notice I breath like that sometimes when I am relaxing. Chunyi Lin cautions against taking in your stomach too far on one of the tapes so I guess there isn't the same range of movement as in deep 'normal' breathing but as I say this seems not to matter. It begins to feel normal and nicely relaxed after a while.
Having a shower before the active exercises has been fine for me. I eat breakfast 30 minutes after the exercises but if I'm feeling hungry I eat breakfast and wait 30 minutes before starting. It seems fine either way. I've wondered about the warm water question myself so I await that reply from others with interest.
Love,
Kev