I agree with
Mänwë's conclusion. This has been a stimulating topic but actual evidence on what Tolkien really intended sorcery in his world to be is unfortunately lacking.
If we have established that sorcery was the manipulation of spirits, we can only suggest how it was done and who could have done it. We do not even know where the original molested spirits came from. lol.
Kudos to
Boromir88,
Radagastly LMP,
Raynor and
Mänwë. You'll are be getting reps from me by providing all the quotes and extractions from Tolkien's letters and masterfully expounding your points. If the main purpose of the books discussion is to ask the questions (sorry about the oath bit) and learn, this thread delivered.
One thing I would like to highlight is that Sauron was actually successful in battles before. He captured Minas Tirith on Tol Sirion in the FA and then the stronghold of Eregion. We know not much on how he performed the second deed but in the first, he used (aha!) sorcery.
EDIT: Not directed at me, but I think a
Song was the medium of spell in magia and that the act of singing it, was incantation for the particular spell to come into effect.