Central problem with your scenario is, if you're assuming a very minimal butterfly like that, then it's difficult to see how Mondale would possibly go against the Democratic caucus on the Gulf; which is what lead a lot of Congressional Democrats to simply give up on that year. (Brown, Tsongas, and Clinton all were not so tainted by such votes)
The fact that the main candidates IOTL were trying to be different types of Democrats wasn't a freak occurrence, you know. Tom Harkin was there in that race as an unapologetic old school Democrat and he just didn't take off. Wasn't the year for that kind of thing.
I'm very doubtful a Mondale who had settled into the Senate for a decade would choose to skip two elections and then give '92 a shot, doubtful still that he would win the nomination even if he did that. Same question as above also applies: just what happens in the meantime due to Mondaler quitting on '84?
It is true that Mondale would probably have voted against the Gulf War, and the immediate consequence would be that it would be assumed he had no chance in 1992. But maybe by late 1991, with Bush's ratings coming down a bit from their sky-high levels and more concern about the economy, he figures he may have a chance, that it may be his *last* chance given his age, and that in any event he still has his Senate seat if he fails (which would not have been true in 1988--unless Minnesota allows simultaneous senatorial and presidential campaigns by the same person). By 1992, the Gulf War was a lot less important in the Democratic primaries (and with the electorate as a whole). After all, Clinton's statement of his own position on the war was "I guess I would have voted with the majority if it was a close vote. But I agree with the arguments the minority made."
http://articles.latimes.com/1992-07-31/news/mn-4566_1_gulf-war If the war had really been important to people in 1992, that would have been as damaging to Clinton as Kerry's "I actually did vote for the $87 billion, before I voted against it" was (or so at least it is widely assumed) to Kerry in the 2004 general election. The truth is that with the fall of the Soviet Union, and with Islamist terrorism not yet a major concern to Americans, foreign policy was less important in 1992 than in almost any other modern American presidential election. It was almost as if the US didn't *need* a foreign policy...
True, Harkin, the closest thing to a representative of old-fashioned liberalism, didn't do very well, but in the first place his populism seemed a bit strident to many people, and in the second place he hardly had the prestige of a former vice-president of the United States.
And by the way, "When asked about the Persian Gulf war, however, Tsongas said he would have joined the majority of congressional Democrats in opposing the use of force."
http://articles.chicagotribune.com/1991-04-12/news/9102020672_1_tsongas-corporate-bashing-democratic Likewise, "On the issue of the Gulf War, Brown said he was against the initial military involvement against Iraq and that Bush should have tried harder to seek a negotiated settlement to the problem. "
http://www.thecrimson.com/article/1991/10/23/jerry-brown-speaks-at-bu-pone/ So yes, declared opponents of the war did announce their candidacies in 1991.
As for what happens in 1984, my guess is that Hart wins the nomination, and loses decisively, though not as heavily as Mondale. Glenn might pick up some of the OTL Mondale vote in the primaries, but he was seen as a bit too conservative by many Democrats, especially on national security matters. I think it's plausible that Dukakis still wins the Democratic nomination in 1988.
I agree that the chance of *all* the things happening that I posit are probably pretty small--but still I think it's the best chance Mondale has for the presidency after 1980.