Thank you for your answers so far.
Ok, so I'll concentrate on the Darkness tree and willpower, then. I mean, I already carry a double-bladed lightsaber with me to activate certain abilities because they do stay active even after changing back to the electro staff.
But what is the reasoning behind this electro staff discrimination? I mean, when you play e.g. a Jedi Guardian you can use ALL your abilities with lightsabers AND vibroswords and if you play e.g. an Imperial Sniper you can also use ALL your abilities with sniper rifles AND blaster rifles -- so why are electro staves not valid for so many assassin/shadow abilities?
Nevertheless, thanks again; I'll try the suggested approach and will let you know how it works out in the higher levels.