Yes it seems so obvious and I *think* I also tried it but... guess I didn't try hard enough :P Because now I succeeded it, and that saved 38 frames :). I had to lose 2 of those frames on the world map so that VD2 didn't desynch, but still a 36 frames improvement, thanx a lot for the suggestion!
PS: I think the reason why I didn't succeed this last week was that I did not yet know about the X/Y trick. So, I wasn't even able to carry a shell up to that point. That must have been the problem. I will now have a second look at my whole video and see if I can abuse the X/Y trick at another point as well, although I doubt it.
---edit---
You can download the newest version of my video
here
It contains the improvement in Vanilla Dome 1 and the normal goal for Vanilla Dome 2. I didn't want to just copy+paste the whole first part of VD2, so I did it a bit different this time, actually 3 frames faster. I don't know yet whether I should take the upper or the lower route; upper route means swimming+jumping upwards, lower route means swimming without shell for some time. So the second part that you can see now is not yet final (of course I'll hit a few more more bats during the flight ;))