1968: Agnew helped and Muskie hurt, LeMay hurt a lot.
This is backwards. Muskie greatly helped Humphrey's campaign and even would allow protestors to take the podium to air their views. Historians agree that the choice of Muskie was a good one. Agnew was a drag on Nixon. Not just politically, but Agnew was a gaffe machine and made Nixon look bad, especially in 1968 when the Democratic ads tore at Agnew's image.
Agnew did the nearly impossible in 1968. He appealed to southerners and Rockefeller Republicans (he was Nelson's suggestion).
Muskie didn't add anything. It was two extreme liberals and there was no real geographic base. A candidate from the west, south (which would have been difficult), or even the midwest would have been better.
Ed Muskie and Hubert Humphrey were not extreme liberals. In your dreamy world, sure. In the real world, they are normal liberals.