Just a thought: That "110+" year ago Romulan incident? That may seem a lifetime ago to us, but not for Romulans, who have lifespans much longer then ours (I don't know their limits but I'm guessing it's a lot closer to Vulcan's 250-300 years then 24th century humans' 140-ish). There may still be bitter Romulans in high places who were friends with that first Romulan Commander.
So? The Soviet Union fell only about 20 years ago, well within the lifetimes of the people in power today. Back then, Germany was split in two, now it's reunited. Whole countries have come into being or ceased to exist, and the people running the countries that exist now still remember the way things were before. It doesn't mean they're all trapped in the past and incapable of altering the status quo, because politics and history are bigger than a few people's resistance to change.
Besides, I'd think that any friends of the "Balance of Terror" commander would be more likely to blame his demise on the warmongering Praetor who sent him on an unnecessary mission of aggression rather than on the Federation. Especially if you go with the John Byrne version from the comics. But given how many governmental purges the Romulans have gone through in the interim, I doubt there are that many of the old guard left (although Sherman & Shwartz have a rehabilitated Charvanek, their name for the "Enterprise Incident" Commander, holding a high post in the 24th-century Romulan government).
There may be living witness Tholians still pissed that Jonathan Archer didn't hand over that time ship in 2152, or that Enterprise didn't piss off immediately after the Defiant warbled off to the mirror universe.
I believe
The Sundered established that individual Tholians have extremely short lifespans, but they have somewhat of a hive mentality. Anyway, it's already well-established in the books that the Tholian government does hold a grudge against the Federation for the events of
Vanguard. Of all the Pact members, the Tholians are clearly the most hostile toward the Federation. But that's balanced out by the Gorn, who have apparently always had amiable relations with the UFP save for two incidents (and whose current regime owes its existence to Picard and Data).
Anyway, even granted that most of the members of the Pact have some history of tension with the Federation, that doesn't mean that's the one and only issue that defines them as governments or as species. From the perspective of the Trek viewer, it seems as if the whole universe revolves around the Federation and that neighboring powers are defined solely in relation to it; but realistically, from the perspective of one of these other governments, the Federation would be just one of many concerns, internal and external, that it had to deal with. The Pact is going to have to deal with all those issues -- their internal economic and social problems, their relations with neighboring nations
other than the UFP and the Khitomer Accord signatories, the effort to balance the priorities and agendas of the Pact's own members, etc. Even if some of their members do have grudges against the Federation, that doesn't mean that's necessarily going to be the singular thing defining their policies and actions as an alliance, because it's just one of many things they'll have to weigh -- including the wildly conflicting agendas of the Pact's own members, their jockeying for status in the fledgling alliance, etc.
Humans appear to have shorter lifespans then the majority of Trek aliens. This Typhon Pact, although only publicized and revealed recently, may have been the life's work of Gorn, Romulan, Breen (how long to Breen live?), Kinshaya and Tholian ambassadors.
Not according to what we were explicitly told about its formation in
A Singular Destiny. It was a reaction to the events of the
Destiny trilogy.
Also, we have no information on the life expectancies of Gorn, Breen, Kinshaya, or Tzenkethi. There is a
Strange New Worlds story depicting the Gorn captain from "Arena" encountering Ben Sisko, but SNW stories generally aren't treated as part of the overall novel continuity.