Good points, except for to me the weak argument based on the second Corbett fight and the Monroe fight. Corbett must have been way past it, and Munroe was hardly up to even being a journeyman.
Fitz may come closer to Dempsey in punching power than anyone Dempsey fought comes close to Jeffries in toughness and chin. I think Jeff actually met tougher opposition. How green was Dempsey for Flynn? He had been fighting three years. How many fights had he had? I think he was fairly experienced and had built up a local reputation. What seems to be obvious about the Flynn fight is that Dempsey was completely knocked out. His version of the fight in his autobiography is nothing like the newspaper ringside reports, except for coming around fighting Bernie in his corner.
"Dempsey wears down Jeffries" This seems to me the least likely outcome. Whatever else, Jeff seems to have been a stamina freak. I don't see him getting worn down. "Jack too fast and explosive for Jim." This is a better argument for Dempsey "The only physical attribute Jeffries matched Dempsey on is pure strength" I would give Jeff the edge on chin. Until the aged Jeff ran out of gas against Johnson, he stood up against big punchers such as Fitz and Sharkey and Choynski. His chin seemed top rate. Dempsey certainly was resilient, but I think his chin might be a notch below. I rate Jeff over Dempsey in an historical sense, as he cleaned out the division up to 1903. There would be no major criticisms of his record if he hadn't ducked Johnson in 1904. H2h I believe would depend on how Jeff fights. If he is passive, he would play into Dempsey's hands and might well suffer too much early damage to outlast Dempsey. If he fights aggressively, moving forward and driving Dempsey back, which I think he could do being the bigger and stronger man, I think he wears Dempsey down and stops him late. There is so little film on Jeff. In what we have, he doesn't show an overhand right, a punch Tunney couldn't miss Dempsey with. A question is how effective would Dempsey's bobbing and weaving be against Jeff's probable body attack? This is a flip a coin type fight, but I lean toward Jeff.
Sam Langford, when asked how Harry Wills (whom he fought 18 times in his career) would do against Jack Dempsey, said in the June 5, 1922, Atlanta Constitution "Well if he ever fights Dempsey my money will be on the present champion. Dempsey is the greatest fighter I have ever seen. He hits twice as hard as Jim Jeffries and is as fast in the ring as James J. Corbett."
Well, personally I don't put much stock in these quotes. Langford is talking to a newspaper in the deep South. He is quoted as saying the white guy is the greatest and would beat this black fighter or apparently any black fighter. This could well be his honest opinion. It could also be a political statement designed to keep him viable among white promoters. Interestingly, does saying Dempsey is better than Wills and likely to win make it more or less likely that a Dempsey-Wills fight will be made? I think more, which could be a motivation for Langford. After all, if Wills got to be champion, Langford might well have been in line for a title shot.
I think you are maybe extemporizing a bit here NB Langford also said Dempsey hit harder than Jeffries and was as fast as Corbett,2 white champions.
I tend to see Dempsey as a watershed heavy, a paradigm changer for the division. In all deference to those who proceeded him, he was a different breed, brought about and fed on a set of new expectations from the audience to bring things to a boil quickly using using his speed and power... and skill. I don't see Jeffries outlasting the assaults over the first few rounds. Of those who came before him, Johnson would have the best chance and I still pick Dempsey. And there is sufficient footage on Jeffries to get an inkling of he liked to fight, especially against a guy who wanted to enforce his power like Sharkey. I don't see the keys in there to contain, let along figure out, Dempsey.