Yes you can flip a switch and direct the power elsewhere, that is what switching yards do. The plant might have a supply contract with Alcoa but if Alcoa doesn’t use the supply they contracted for you they don’t have to throw the power away. Usually those agreements have take or pay provisions which require the buyer to pay regardless of whether they take the power. If they don’t take the power the generator is free to sell it on.

Transmission losses are not nothing but they are not huge. Maybe 2% to New York (it isn’t they far). Maybe 5% total if you include distribution.

Edit: Follow the lines out of the plant, at 44.978432, -74.797925 they split.