Its a question about timing. Teldrassil corruption have happened during Classic. But quests in Silwermoon are BC. Does this timeline is just a game mechanic or not?
Also, Blood Elves had dropped to Night Elves lands together with Draenei. Those Blood Elves were taking orders from the main ruler of the Blood Elves (at those times Blood Elves of the Silwermoon were still loyal to their prince!), and they were hostile to the Alliance, while Draenei became part of the Alliance.
Again, question in time line. If that have happened before quests in Silvermoon, than its pretty logical that Alliance have send spies instead of helping hands.
Not the Horde, only Sylvanas. And just to use them as arrows against the Lich King: