While a Wilson victory is still possible, I'm giving Taft the advantage.
IMO you are *wildly* overestimating the appeal Taft could have with TR voters. Taft was simply a very unpopular president. That is why the GOP lost control of the House in 1910, *before* the Progressive split. In fact, one reason some Republicans tried to get TR to run in 1911-12 was precisely because they thought Taft's electoral prospects in 1912 were hopeless. (As David Sarasohn notes in his *The Party of Reform*, p. 149, the few elections held in 1911 showed further Democratic gains.)
And if anything the assassination of TR would make his followers *more* bitter about Taft. Remember that Taft had said of his progressive critics that "With the effort to make the selection of candidates, the enactment of legislation and the decision of courts depend on the momentary passions of a people necessarily indifferently informed as to the issues presented, and without he opportunity having been given them for time and study and that deliberation that gives security and common sense to the government of the people, such extremists would hurry us into a condition which could find no parallel except in the French Revolution or in that bubbling anarchy that once characterized the South American republics. Such extremists are not progressives; they are political emotionalists or neurotics...."
http://news.yahoo.com/february-13-p...neurotics-mysteries-providence-010350867.html I do not doubt that some TR supporters would blame words like this for the assassination. Even those who didn't would be bitter at Taft for "stealing" the GOP nomination from TR.
For that reason, while he would not get as many votes as TR, Hiram Johnson or whoever the Progressives would nominate would get a substantial vote--as a protest against Taft and a symbolic gesture of support for TR's memory. Many other Progressives would stay at home. And yes, a substantial number would vote for Wilson as the more "progressive" of the two remaining major candidates (and the one who had not said anything as personally offensive toward TR as Taft's "neurotics" remarks were interpreted). No doubt some would vote for Taft but not nearly as large a proportion as you think.
One piece of evidence that the Taft and TR vote could not be combined behind one candidate (least of all one as unpopular as Taft) is that in some states the Republicans and Progressives *did* run "fusion" tickets for governor and other offices--and the fusion candidates almost always did considerably worse than the combined TR and Taft vote. in Kansas, for example, Taft and TR combined got 53.4 percent of the vote and Wilson only 39.3%
http://psephos.adam-carr.net/countries/u/usa/pres/1912.txt Yet the Democratic candidate for governor, Hodges, narrowly defeated the Republican Capper (46.55%-46.54%), even though there was no Progressive candidate.
https://books.google.com/books?id=ksBiaAS8jXoC&pg=PA1
In fact, Louis Bean has argued (in *How to Predict Elections* [New York: Knopf 1948]) based on congressional returns of 1912, that the Bull Moose vote for president would have split evenly between the two old parties--which would have given Wilson almost 60 percent of the two-party vote!
https://archive.org/stream/howtopredictelec00inbean#page/68/mode/2up I would not go so far. But it must be emphasized that Wilson would not have *needed* one-half or even one-third or even one-fourth of the Bull Moose vote. One should not ignore that while the majority of Bull Moose support was undoubtedly from Republicans (albeit Republicans who would not necessarily vote for Taft in 1912) they did have support from a few prominent Democrats like the famed Tammany Hall orator Bourke Cockran, and Judge Ben Lindsey of Denver, and no doubt some rank-and-file Democrats as well.
(One other point: In OTL some of Debs' vote was from voters who were advanced small-p progressives but not committed Socialists. They felt it safe to vote for Debs because a moderately progressive candidate--either Wilson or TR--was considered sure to win. If it suddenly became a choice between Wilson and the "reactionary" Taft--Taft's image as a reactionary may have been unfair but it was a common perception-- many of them might vote for Wilson. After all, the reason many 1912 Debs voters voted for Wilson in 1916 was not *solely* the peace issue or the fact that the Socialists had nominated a less charismatic candidate in 1916. There was also the fact that Hughes--in the words of William Allen White, who supported him!--" talked tariff like Mark Hanna. He talked of industrial affairs like McKinley..."
https://books.google.com/books?id=yXpNAAAAYAAJ&pg=RA10-PA6)
I have in the past argued that Wilson would win a one-on-one race with TR in 1912, but I'll admit to some uncertainty on that. I think it *much* less likely that Wilson could lose to Taft one-on-one--and the race we are talking about is not even one-on-one, since the Progressives will still exist and will get *some* votes from those who revered TR.