Christ is Risen!
I'm not Orthodox, but my opinion......
The analogy only makes sense if you view the two saints as standard bearers of mutually opposing camps. I would say the following:
1. "all" sainthood means is that God has revealed that the person is in the dwelling place of the Saints, the bosom of Abraham and numbered among the righteous, as evidenced by the veneration of the people (generally due to miracles) and through the authority of the church (traditionally through the local bishop. For better or worse, for those in union with Rome, that authority has become a formal process in Rome since the 1500s).
2. This is not the same as politically lionizing someone for a political or sectarian goal (e.g. there's a big difference between Sainthood and the way, say, Communist Parties lionize their "greats")
3. As a bishop at a council, St. Mark had every right to oppose the decree of the council if he thought it was heterodox. And since the council's result was ultimately rejected, often vehemently, by the people and the other clergy, it's not as if he was the only voice in the world who was opposed. So one cannot really lay the blame for "failure" of "reunion" on him. And as far as judging his work....I kind of doubt a critical edition of his full work exists in Greek, and what exists in English is appallingly small and used for polemical purposes. I would defer on evaluation of his work, how accurate his evaluation of Latin theology presented at the Council was (and to what degree the Latin theology presented at the Council stands critical Catholic theological scrutiny today) to qualified theologians. This is usually done by people with doctorates, and often yields odd results for polemics*.
4. as far as St. Josaphat, I really don't know much about him (and don't wish to really tread there since I'm not Ukrainian). I would, however, say that the sectarian/political/cultural pressure wielded on the (what one Ukranian catholic historian described as a culturally weaker) Kyivan church from the Polish crown/intelligenstia, the Counter-reformation Latin Church, the Calvinist sectarians (who I'm told were an influential part of the Polish elite at the time), and from the Muscovites was immense. This yielded fierce divisions in the Kyivan church, which combined with political passions would inevitably result in a very bad religious environment.
Finally, the answer to how should ECs view St. Josaphat.... they should view him however objective history can view him. (and yes, there is such a thing, or at least approximation, and a good historical research does exist). In my experience, objective history usually knocks people off of hagiographic pedestals (since hagiography's purpose is different from objective history), presents evidence pro and contra (and often shattering) to whatever paradigm one might be looking for, and often has abundant amounts of weird data that show the reader how differently people of the past think compared to people of today.
*e.g. St. Gregory Palamas, the defender of "Eastern Orthodoxy" against Barlaam the "western scholastic" according to the ridiculous caricature that's been out there for so long, quotes Augustine (without attribution) far more often than his opponent. [this was in an article in SVS theological quarterly, I can find a citation if one wants]