Sam, welcome back. I agree with most of your points. An overnight New York-Pittsburgh train might have some success, much as the Washington-New York-Boston Northeast Regionals (65-66-67) that run overnight do. They call at poor times at most stations including New York, but ultimately could use sleepers for the longer distance travelers. Maybe a sleeper on an overnight train as well New York-Pittsburgh.... and even a set-out sleeper originating from Philadelphia that would be quickly coupled to the train along with the obligatory engine change at 30th Street Station.
As far as slots are concerned, as you mention with decreased freight traffic I don't think this should be an issue. East of Harrisburg, a new train could run in place of one of the Keystones, and in the case of an overnight train, with Amtrak as the owner of the property, time slots are not an issue except maybe in the early morning when an eastbound would be travelling the Northeast Corridor into New York.