I've tried a mouse-only concept and I think it works quite nicely.
It may take you a minute or 2 to get used to this different control scheme, but I like it.
By moving the mouse you move the knob and thus the player. When you hold down the right mouse button, the potions and the player-switch-icon appear. Move the knob next to one item and release the right mouse button to activate it.
Currently, only the player switch and the red potion ( = gravity change) are usable.
I've uploaded the new movement prototype to George's SkyDrive account, as usual.
Please tell me what you think, what you like and dislike!
EDIT: There's a new problem: For now, it's impossible to pull crates because you would have to use another key. Anybody got an idea?