I think there's a major fork between the TOS history and the TNG/Enterprise history. "First Contact" essentially blew most of the unwritten/unspoken pre-Federation history out of the water.
In the TOS episode "Metamorphosis" Zefram Cochrane was referred to as the "discoverer" of the space warp. It's not the "Human" discoverer of the space warp, but the inventor pure and simple. That jives with a lot of the other background that is never explicitly spelled out but is fairly obvious. Here's some examples:
In "Balance of Terror" Spock refers to the first Romulan war as being fought "By our standards today, with primitive atomic weapons and in primitive space vessels" - which kind of puts the Romulans and the Humans at parity. Cloaking, for the Romulans, was unknown. According to Spock, all of the fighting was ship-to-ship. Which makes sense if it's a border war that never got close to the home worlds.
In TOS, not many Vulcans serve in Star Fleet. I think Spock was the only one we ever saw and the only others we heard about were on board USS Intrepid - an all Vulcan crew on a ship with an English name. Hmmmm....doesn't sound like the Vulcans are leaders, but more like they're clients of the Humans.
When we see the Klingons in TOS, they don't regard the Humans as being newcomers, or primitives, but pretty much equals.
So, to me, the unwritten/unspoken assumptions under TOS were that Humans were one of the first to get warp drive, that they probably went and found the Vulcans and that warp technology may have even been a Human invention that spread around.
"First Contact" completely blew all that out of the water. Now, the Vulcans are the elder race, Humans are the n00bs. Following on from that, Enterprise established that the Romulans are more technically advanced than even the Vulcans and the Klingon Empire was vast and old long before the Humans burst on the scene. Andorians, Tellarites, Orions, Ferrengi, just about everybody is way, way ahead of the Humans.
To me, Spock's synopsis of the Earth-Romulan war (along with many, many other things in TOS) fits with that unspoken canon above but First Contact and Enterprise do not. So, the question becomes, how do you reconcile the new, explicit canon with the old, explicit canon that was based on the unspoken canon?
Where I'm going with the Earth-Romulan war, is to take this Romulan penchant for secrecy and use it. For most of the war, no one on the Coalition side will even know that they're fighting the Romulans. Any real Romulans found will be dead and assumed to be rogue Vulcans which is going to stir up a lot of mistrust inside the Coalition. And, when the war breaks out into the open, it's going to be massive and there are going to be strikes that will really push many of the older races back from their positions of superiority. The battles, though, will be fought in space, or as strikes against planets with WMDs, not ground actions so there will be no face-to-face contacts. And the Romulans are not going to be happy campers at the end and will be left in a position of weakness but with enough strength to force the treaty with the Neutral Zone. A small group of people on the Coalition side will know the truth, but will hide it to keep the Coalition together because otherwise the Vulcans would probably get kicked out on their collective asses.
This is fun, now I want to go finish off a few chapters. Blah, I need to work instead