Eugene McCarthy would not have launched an independent candidacy.
Why not? I'm not too familiar with McCarthy, but given that Nixon appears to be the least pro-Vietnam-War candidate in the race (which is saying a lot), why wouldn't he make a run? Party loyalty? --McCarthy ran as an independent in '76 (when Vietnam wasn't a factor), and supported Reagan in 1980 if I remember correctly, so it's not a huge hurdle to break ranks.
If McCarthy did run as an independent, what sort of an effect would that have? Would he have any chance of winning states, or would he solely serve to make sure Nixon gets 400 EV Wilson-style?