If Mitt Romney endorsed Phil Johnson would that give him the 15% support required to get into the debates And if Johnson got into the debates who would he hurt the most Clinton or Trump
For the first question, probably not. But the second question is very interesting. I could argue both sides, but probably more convincingly that he would hurt Trump more than Clinton.
Although I perceive the Democrat vote to be softer this election than the Republican vote, I don't see as many soft Democrats switching to the Libertarian party as soft Republicans who might. I think Gary Johnson is more ideologically similar to Clinton, but traditionally the Libertarians are closer to the Republicans. Most voters identify along party lines first, and then later evaluate their candidate and the other candidates through a lens of confirmation bias. This means that more Republicans than Democrats would entertain the thought of a Libertarian ticket.
Now, in theory, this could work both ways - the assumption above is that Johnson would win votes, as opposed to lose them. Libertarian voters would also be more likely to consider going Republican than Democrat. But at the end of the day, I just don't see Johnson having that many votes to lose. As with the Green party in Canada, expect the Libertarians (and U.S. Greens) to poll well until election day but to underperform at the ballot box as these voters will break for one of the major parties. The only question is who the voters who were parking with either of those minor parties end up breaking to.
The Democrat campaign is out of the Clinton's control. The Republican campaign is almost entirely in Trump's control. Let me be very clear in my meaning here. There are many things which could negatively effect the Clinton campaign and there is nothing that can be done by the Democrats to stop it (snarky aside: maybe they manage to off Assange and get lucky on the other stuff). Any one of the scandals that are currently circling Clinton and just barely being held at bay could sink her campaign. Conversely, the only thing that can sink the Trump's campaign is Trump himself - and it's looking increasingly unlikely that he will.
Under such circumstances, Trump is much more likely to get more of the soft Libertarian voters - especially since he better represents Libertarian values than Gary Johnson does. Anything that prevents those voters from switching to a major party is a relative loss to Trump, who will get the majority of the switchers. So, presuming a reasonable debate performance by Gary Johnson, his inclusion in the debates would probably hurt Trump more than the Clinton.
If it seems strange that I haven't talked about either Clinton or Trump's performances, it's because they probably won't matter. If they do matter, it is Trump who will benefit from low expectations for himself and high expectations for Clinton. Even after all the evidence to the contrary, people actually believe Trump is stupid. There are a few foreseeable ways Trump could overwhelmingly win the debates, and the more time he and Clinton talk the better (for him). A third party candidate of any stripe will reduce the scrutiny of Clinton and reduce his opportunity to exceed expectations.
I'll ignore any conjecture about the health of the specific candidates and how Johnson's presence would benefit any specific candidates who might have less than stellar health as I don't want to stray into conspiracy theory territory. But if, only hypothetically speaking of course, a candidate was sick and had trouble...I don't know...speaking or standing for extended periods of time (for example), then that (hypothetical!!) candidate with the (theoretical) health issues might benefit from an additional candidate who would both draw attention away and who would reduce the amount of talking each candidate was required to do.
So most likely, I would expect Johnson in the debates to hurt Trump more than Clinton.