3. The N train in comparison to the 7 train, which only has four Manhattan stations almost all along a single street, has 12-14 stations in Manhattan hitting almost every major destination (Upper East side, Herald square, Union Square, NYU, NOHO, SOHO, Canal Street, City Hall, World Trade Center and the Financial district).
While I am not disagreeing that the N train shouldn't be extended, according to the subway map directly off the MTA website, the N train only makes six stops in Manhattan, and does not go all the way to the financial district and the world trade center as the N train branches off the rest of the line at Canal St and goes over the Manhattan bridge back into Brooklyn. That being said, if the N and the W were both extended at the same time, the W train provides access to everything you have stated.
As far as the 7 goes, it would make much more sense if the Flushing Elevated had four tracks, so you can run express trains in both directions at the same time making this connection much more viable. (Maybe as an 8 train or something). Interestingly enough, the 7 does provide the cross platform transfer to the N and W at Queensboro Plaza.
Also, the LIRR connection would be only to the Port Washington Branch as it is the only line serving the Willets Point station. If you wanted to improve this, maybe find a way to link the JFK airtrain system to the LGA one via Flushing Meadows, and you can kill two birds with one stone. Direct access to all LIRR branches, and also a one seat connection between LGA and JFK. (And yes, I have seen many airlines that unfortunately sell overseas itineraries that have the domestic leg fly into LGA and the international one out of JFK).