I think that without resorting to errata you have to accept this stuff as one of the intrinsic limitations of cube. There's been a lot of talk about tribal on these boards, and if you wanted to do it well at a high power level your best bet is probably designing (cards/a set) from scratch.
I'm really not sure where this number comes from.
Vampire Lacerator is perfectly playable.
Highborn's biggest drawback is that most cubes want to use their color-intensive slots elsewhere (Bloodghast says hello). At 1B Highborn would be far more appealing design-wise.
That's a cool card I didn't know about, but I do worry about including a BBB spell in a multicolor focused cube. I already have a couple Swamps matters cards, and really don't want to push further in that direction.