I personally doubt Sylvanas trusted Lord Godfrey that much- it was more the level of audacity it took to assassinate her. The Forsaken are NOT slaves of Sylvanas- she's just the only one who will protect them against any threat. My best guess is that the undead are under control of the Valkyr as long as the Valkyr choose to exert that control. In essence, Sylvanas expected that Godfrey's hate would overpower any personal desire, she just misjudged how quickly he would switch once his hatred was sated. http://bit.ly/i97Z51

Not all of Silverpine, but the portion just north of the Wall did use to belong to Gilneas (and Crowley). Shadowfang Keep, Pyrewood Village too. And at least some of their owners are still alive, so they do kinda have a claim to it still.
http://bit.ly/g3Ily9

It's his land. It's Gilnean land. The Forsaken are invaders out for 'unliving space' as it were. They're thieves and murderers who use plague and val'kyr to slaughter the inhabitants and turn them against their own people. It's very simple.
http://bit.ly/eyK2ez