Although this is veering off topic, I think it's a perfectly reasonable interpretation to say that the events of ENT exist in an altered timeline. In fact, we've seen on-screen evidence of this. And this helps smooth out some of the continuity issues between TNG and Voyager as well.
Assume that TOS as documented in the episodes is the original, unchanged timeline. It holds all the way until First Contact. Then, due to the Borg, the Enterprise-E and its crew goes back in time to the point at which humans made first contact with aliens. The Enterprise crew gets actively involved with Cochrane and makes him aware of things he shouldn't have been aware of. The Borg sphere is destroyed above the Earth and pieces of it crash land and eventually produce Borg drones on Earth much earlier than any contact with the Borg was ever supposed to happen.
Thus, when we see the events depicted in Enterprise, we're seeing the altered timeline with events unfolding differently than they did before. Also, when we see Voyager's seasons involving Seven of Nine and we hear about Borg history, and contact between the Federation and the Borg that predates Q, Who?, we're also seeing that altered timeline, since those seasons of Voyager happen after First Contact.
To me, that all makes much more sense than trying to get all of the differences between TOS, TNG, Voyager, and Enterprise to all fit together.
