Maybe not. That might be why he was happy taking two (or three, depending on where Steed is) junior wardens in exchange for the vamps he lost in that fight.
Who said it was evocation at all? Could be a thaumaturgical spell requiring a full blown ritual for Harry to pull off.
Hardly junior anymore I think. Most of the senior wardens have been killed, no? In any case it isn't a great trade I would think - experienced Blampire wizards for less experienced mortal wizards. Jim always goes on about any surviving Blampires are the most ruthless, pragmatic and wiliest of their kind. Although I will admit that rather didn't show in the scene...
It would hardly be a very useful spell if it wasn't evocation. I suspect any thaumaturgy Harry could muster would be dangerous enough to them anyway, regardless of the spell Kemmler created.
Jim implies you just have to be very unlucky.. I'm wondering if it's simply being Starborn is unlucky or he specifically meant being an immortal Starborn is unlucky. Drakul did end up stuck in a mortal(ish) shell after all.
Interesting idea. My take is that he meant all starborn - immortal or mortal. No one seems to regard it as an especially good thing despite the fact it makes you resistant to Outsiders and more able to banish them, among whatever other abilities and traits come with it.