The point of the to Euclid is not really for Broadway access but to increase service that is currently limited by the Cranberry segment- sending the to Hoyt after Whitehall is the easiest way since you have provisions and Bay Ridge is fine with losing direct Manhattan local service since they transfer anyways. With the City Hall curve you end up with 24 tph of service which I'd say is good considering future development along the corridor. You'll also be able to have Fulton be less isolated with the transfers downtown. From here you can also get some decent service to all the branches in Queens and encourage development along areas like Aqueduct since it'll now have halfway decent service. Can you do the same with extending the ? Sure, but it'll be much more expensive since you'll need to tunnel through several streets in Manhattan since WTC is too close to Cortlandt for the track to be extended. Note: this would be in combination with the plan and a deinterlined 34th-Broadway.
As for 59th, I'm with you on that. Trying to deinterline CPW presents issues of higher crowds at 59th and also screws up runtimes- sending 6th via the local doesn't help the since the only other true express segment would be 4th and sending 8th via the local would create runtimes that rival the worst bus routes (Inwood/Washington Heights to the Rockaways via CPW local is painful slog and it's not like you can swap it out for the since the ride to Jamaica would be terrible too).