I must at first admit that I have had my eye on this project since it started, but I have been consistently.... delayed.... from putting my thoughts to paper. I also feel that I must explain the philosophy of the criticisms that follows beforehand; while I hold that all timelines are inherently plausible on the basis of unknowable variables, the best stories are those that adhere to a strict sense of.... well let us call it "strict" plausibility, where the variables in question are ones that can be observed and understood. When someone is capable of visualizing the equation and operating the mathematics they are left with fewer questions as to the "how" and, therefore, tend not to be as skeptical as to the concept as a whole. Now this isn't absolute, but in my experience that tends to usually be the case.
Now my own criticisms may be subject to innate biases, though given the time period I don't believe it should be an issue, and I won't claim to know more than the next. Take from the following what you may.
Now then, let us see:
So this is problematic in several different ways. While Herbert Hoover was indeed popular amongst some Progressives, he was actually incredibly weak outside of the intellectual circles, and his own campaign was mortally wounded when he opted to try and defeat Hiram Johnson on his home turf. Hoover also fairly definitively threw in his lot with Harding after the Republican Convention, endorsing him and actively decrying the movement to establish a Third Party. He doesn't much fit the profile that you are looking for.
Most at the time were united around the idea of nominating Senator Bob La Follette, least those that weren't Georgists, but there were discussions that also revolved around the possibility of nominating either North Dakota Governor Lynn Frazier or Henry Ford. Obviously La Follette would be the strongest candidate of those that had been considered and the most likely in those scenarios where the Committee of 48 actually succeeds in putting together a ticket and a platform, but the caveats to that are you wouldn't have a split amongst the Progressives, and there is a strong possibility that the Socialists might withdraw their own ticket and fuse with the Progressives.
The alternative to that then is to nominate Henry Ford which would have caused .... well quite a scene. The Socialists would still be running their own ticket as they'd never agree to endorse someone like Ford, and it is actually conceivable that Hiram Johnson might bolt given Ford was a strong supporter of the United States joining the League of Nations, least at the time anyways.
As for running-mates... well I suppose I'd throw Kansas Governor Henry Justin Allen at Henry Ford, though that is in part because I am not sure what Allen's views are in regards to the League. Hiram Johnson has a slew to pick from given the number of Progressives that made up the "irreconcilables", but I think that Nebraska Senator George Norris fits better in a regional aspect as opposed to William Borah.
<Democrats and Prohibition>
Now this has its own issues. William Bryan was indeed nominated by the Prohibition Party in it's first go around that year, but for him to realistically accept that the Democrat's would need to nominate a "Wet" ticket. As you can imagine that is a tall order, and I've tried to wrap my head around it. The closest I could get was if Mitchell Palmer were to ally himself with Charles Murphy and endorse a "Wet" plank, which in this case would have been allowing light beers/wines/ciders, as well as a plank advocating for the recognition of the Irish Republic. The problem with that calculation though is that while Palmer would gain a number of delegations in the Northeast, especially important ones like New York and New Jersey, he is just as likely to lose delegations in the South and West that were "Dry", in this case the biggest loss being Georgia with 28. That means that Palmer would be in the ~380 range even taking all this into account. Assuming that he can make an arrangement with Cox and some of the other Northern delegations (Indiana and Connecticut) that would put him in the ~500 range.
There would need to be some sort of stampede in Palmer's favor, which actually is what the New York gambit was hoped to trigger, and I may be undercutting Palmer's support slightly as some delegates might care less whether the platform is "Wet" or "Dry". The "Two-Thirds Rule" for nominations would have to go so as to then allow a nomination by majority, however its repeal would in and of itself require 2/3rds of the vote in favor so.... well there's the rub. I'd argue that it manages to get repealed as a desperation measure, and indeed there were talks about repealing it in both 1920 and 1924; both times it was nixed by delegates favorable to McAdoo.
Ironically, given the amount of influence that Charles Murphy would have had in the nomination battle, it is virtually certain that Franklin Delano Roosevelt would have ended up being nominated for the Vice Presidency, partially in an effort to mollify those who had favored McAdoo (Roosevelt lead the minority of New York delegates who favored him), partially to add.... star power for lack of a better term; they thought that the Roosevelt name would gain them an advantage.
Now from this point, William Jennings Bryan running for the Prohibition Party nomination makes sense as he was for strict enforcement of the 18th Amendment, and while the Republican Party was in sympathy with him on this he could never throw his lot in with them. An argument could be made for Bryan going over to the Progressives as his name was mentioned in some circles, but he was strongly in favor of the League and might have seen Ford's nomination as a coup of sorts, leaving each faction as incompatible with his beliefs. On the topic of a running-mate, while Billy Sunday was also nominated, he actually had endorsed Harding earlier that July and wouldn't run. Given the conduct I have seen in the records of the Democratic Convention, I would strongly suggest Richmind P. Hobson of Alabama in his stead.
The Socialists would be a mite stronger given the divided field, but a lot of damage had already been done amongst the schism that cut the Party in two between its Anti-War and Pro-War factions, and much of its potential voter base would be drawn in by the likes of Ford, Johnson or Bryan. The idea of the Wilson Administration pardoning any Leftists is unfortunately laughable considering how Wilson himself reacted when such petitions were put before him, and truth be told there was not much public sympathy for the Socialists in the first place given their perceived connections to the Bolsheviks; for many, while prison time was a tad much, they didn't mind them being censored from the political arena.
......
Now you'll have to forgive me as, while I have more to say, this has taken a while to write up, and I need to concentrate on other matters for the time being.