Coherent in what sense? Prioritarianism is likely more intuitive, but a problem is that there are infinitely many ways to draw a concave welfare function and no good reasons to choose one over the other. I don't think negative utilitarianism is necessarily incoherent by itself, it depends on the way it is formalized.
I agree that prioritarianism has the problems you mention. I note that negative-leaning utilitarianism (though not strict negative utilitarianism) has analogous problems: just as there are infinitely many ways to draw a concave welfare function, so there are infinitely many exchange rates between positive and negative experience.
I've always been more of a theoretician, but it's important to try one's hand at practical problems from time to time. In that vein, I've decided to try three simultaneous experiments on major Less Wrong themes. I will aim to acquire something to protect, I will practice training a seed intelligence, and I will become more familiar with many consequences of evolutionary psychology.
In the spirit of efficiency I'll combine all these experiments into one:
She's never seen Star Wars or Doctor Who.
She's never seen David Attenborough or read J. L. Borges.
She's never had a philosophical debate.
She's never been skiing.
Never had sex, never been hugged or even been licked by a dog!
She has so much to look forwards to...
(Though she'll be very boring for several months yet!)