No way Sirius is a dementor....it just isn't going to happen. Whatever lays beyond the veil is going to be much bigger, and much graver than that. Sirius is not PHYSICALLY coming back...that was said by JKR somewhere, although I'm not sure. But he will be back b/c he IS an important character.
JKR also said that the Dursley's were all muggles, so that definitely crosses out Vernon and Dudley having magical powers. As for Petunia, she is definitely hiding something, but JKR said that she wasn't a squib and therefore did not have magical parents...Lily was a muggle-born like Hermione.
I can't see Dumbledore dieing until Book 7, but who knows, it is always a possibility. It is a war, and with any war there will be casualties.
As for Pettigrew killing Lupin....I'm highly suspect of this theory. I agree it is the perfect set up for a confrontation, but I don't think Lupin is going to parish from it. Maybe Snape will save him...