So, how many Nader supporters here would have voted for Hillary or Obama lacking a Nader candidacy?
In order to solve this problem you need to break down the population into groups.
The first group always vote for the candidate of their choice, for example: "My grandfather voted Republican, my father voted Republican, my son votes Republican, and my grandsons will vote Republican too." The second group vote against a particular candidate, for example: "I'm not voting for Clinton ... a damn liberal!" The third group looks for the winning side and votes for that candidate, for example: "At the last moment I voted for Bush and he won!" The fourth group stays home because they don't find anyone appealing, for example: "They're all a bunch of damn crooks."
If you remove Nader, the first group will stay home and become nonvoters. The second group, the ones that always vote against the Democrat and always vote against the Republican, will simply switch to another third party candidate. The third group, if sane, will only vote for Democrat or Republican. So they don't count. And the fourth group we can eliminate because they don't vote for anybody, not even Nader.
So, eliminating Nader from the picture does not in any way, shape, or form help the Democrats. Indeed, getting rid of Nader might very well make things much worse for the Democrats. Much worse than they can possible imagine.