Snape felt his blood bole for quite a few reasons one was for ware dubledoor was sending harry, secondly was the horrible excuses for why it was that particular place and lastly was harrys so called "discuses".
Of cores the westlys couldn't be used then Harry would tell them abut what happened and expect serten type of reaction and if he didn't he wold become suspicious and dubeldoor would lose his spy.
The Grangers were out because they "Did not know the importance of keeping harry on the side of light". They Would do as any deasent people wold do and demand the Dusles (and Dumbeldoor) Head on a plate.
As for Hogwarts Surprisingly not all teachers were members of the order. While they supported Harry's actions over the years they weren't strict Voldermort must die for the good of the world and Dumblerdoor should be supreme ruler of the world.
Grimly pale was out because of some small sympathy for harry so Dumbeldoor was out of options except letting harry stay with him or sending him to this unknown relative. Snape knew Dumbledoor did consider levying him with his other aunt but if he did Harry wold have been independent and power full so it was possible dumbledoor wouldn't have been able to manipulate him as easy.
But now he wold send him to an unknown country and a family that for the most part probably didn't evan want him. Of course their dislike wouldn't go to this level but harry would be able to sense he was unwanted. Besides that they wold be to busy to worry about his visions sense they didn't have any relevant to their problem.
Lastly Dubeldoor changed harry into a girl so that the powers he inherited from his Mothers bloodline would be unlocked so than he could use not only wizard magic but all so the demonic and arciilic magic from his mother so most likely he would be the only known person with this ability. Also the deegans were fame for their raw magical power. In present years there had been one extremely power full necromancer, The chapmen of balance and a white mage with unparalleled helling gifts.