How can I remove a node from a way, without removing the node alltogether? It is still part of other ways.
According to http://wiki.openstreetmap.org/wiki/Potlatch_2/Shortcuts try Shift-J
then drag one of the duplicated nodes a bit to get separated from the other node, and delete the one you want.
Yes, but that was by using another shortcut: - (but not the - on the numeric keypad)