Uh, he resigned his commission, and Virginia seceded...there was no way he could have committed treason against the United States when they weren't a part of it.
You need to look at the historical perspective and see that before the South was invaded and subdued, there were states that were united...thus, united states. The individuality of the states was quite clear, as Robert E. Lee felt loyalty to Virginia, not to some "Confederacy" when he resigned his US commission. Recall that the confederacy formed after states had seceded.
New England states considered secession and raising an army. Again, why was the discussion of that, and the voting on it, not seditious?
Only if you accept the southern acts of unilateral secession to be legal.
New England states considered secession and raising an army. Again, why was the discussion of that, and the voting on it, not seditious?
The Sedition Acts had been ruled unconstitutional some time before so merely talking about rebellion wasn't illegal. Too, the Hartford Convention never seriously debated secession. Those attending advocating the breakup of the Union were voted down early. If you read the declaration issued by the convention you would find that nowhere does it threaten secession.