First off, I think Paul would send Seth there to spy for him. I'm not quite sure in what context; I think Paul just wants to know what is happening at the school; it would alarm him to have Death-Eaters openly there. He'd be worried about student disappearances, worried that Harry, Hermione, and Ron weren't there...He just wouldn't like the situation, at all.
I can imagine Seth being both angry at Snape and bewildered--because murder just wouldn't fit into his concept of Snape. He'd want to know why Snape killed Dumbledore. And, of course, Snape wouldn't give him a straight answer.
That, however, might well convince Seth that more was going on than met the eye.
My computer time is about up, so I'll have to leave in a few minutes. I'd love to do more with this idea, but I think, for now, it's pretty much just a mental exercise.