I personally think ENT is AU because of all the TCW shenanigans.
Compare episode TNG season 4 x 15 First Contact with ENT 1 x 01 and 02 Broken bow. In the TNG episode Picard says that first contact with the Klingons led to decades of war. Yet if you watch Broken Bow, there is nothing resembling a preamble to war.
*Interfence by the Suliban and Future Guy can be blamed for the discrepancy of the history Picard recites and what we actually see in the epiosde Broken Bow.*
In ENT 3 x 11 Carpenter Street, Daneils tells Archer that history doesn't mention anything about a conflict between humans and Xindi. Quoting Daniels to Archer: "The events that are taking place are a result of temporal incursion. They are not supposed to be happening".
ENT 3 x 18 Azati Prime, Daniels shows Archer the war between the Federation and the Sphere builders in the 26th century. Daniels lays out the motive for the Sphere Builders meddling with time. The Sphere builders are on the losing side of a war with the Fed and Xindi. The Sphere builders decided to use time travel to usurp established history and use the Xindi in the 22nd century to destroy Earth. This would be before Earth and the Xindi had ever made first contact and before Xindi joined the Fed.
*The of meddling by TCW factions can be used to explain why we the audience and characters in TOS-TNG-DS9-VOY never heard of the events and aliens that we saw take place in the first 3 seasons of ENT. To borrow a line from Doctor Who, "Time can be rewritten". That is exactly what we see happening in ENT. This (and other things) IMO should classify ENT as an AU series.
With regards to the Borg, FC, Regeneration, Year of Hell and Relativity thing. *deep breath* Think about it this way.
Regeneration is obviously a sequel to FC. There are no problems there. Time travel adventures can leave things behind in the past. Similar to the arm of the Terminator in the 1984 film and the sequel T2.
The problem is 7 of 9's statement in the episodes YOH and Relativity. Where she says the Borg were present during the events of FC. Ignoring that episode Regeneration and ENT the series had not been conceived yet. I maintain there is no way 7 of 9 can know that. The dish the Borg were constructing to contact the collective in 2063 was destroyed by Picard and Worf BEFORE they could send a message out. You can literally watch FC and then YOH and Relativity and scratch your head wondering HOW 7 of 9 knew that. It has been said before that Regeneration's inclusion "fixes" the gaffe. However at the time when Relativity was written, 1999. It's obliviously a mistake by the writers in an attempt to sound meta. They should've gone back and actually watched FC before referencing it.
Other gaffes in Relativity include"
The first time the Doctor was activated. The Doctor himself says he was first activated during the Caretaker situation, in the episode 'Projections'. 'Relativity' contradicts that.
In "Future's End', Captain Braxton says he has no memories of being stranded in the 20th century. Yet in 'Relativity' he says he does remember that life.
In 'Caretaker' Janeway says she wants Tom Paris to help her find the Maqui. In 'Relativity' it's changed to wanting him for his piloting skills. Despite him never piloting the ship in the Badlands. VOY had a pilot in Caretaker episode. She died because of the displacement wave transporting VOY to the DQ.
These are continuity errors present in one series complied in ONE episode. Were the writers and producers too busy to actually watch the episodes they were attempting to reference?
Lastly the pogo paradox/predestination paradox thing applied to FC via Relativity doesn't hold water for me.
Compare time travel in FC to time travel in Terminator 1984 for example. The machines//skynet send a Terminator back in time to kill Sarah Connor before she gives birth to John Connor. John Connor sends his own father, Kyle Reese in to the past to protect, fall in love, and have intercourse with his mother. Kye also gave Sarah massive amounts of spoilers about the coming war which would ensure she would train John. This would ensure that John himself would be born to fight the machines.
This is a causality loop. Kyle Reese does not exist in 1984. He hadn't even been born yet. If he never time traveled to the past, then Sarah Connor wouldve died and never gotten pregnant with John. Also if the machines/Skynet had never sent that first Terminator in to the past to kill Sarah, then John Connor of the future would've never sent Kyle Reese to the past. The future John created himself via casualty loop in a sense. While the rise of the machines would be explored later in T2 and T3 with use of more time travel.
Compare to FC where Zefram Cochrane was already planning to conduct his warp flight on schedule. The then Borg show up and act like time traveling saboteurs on Cochrane's camp and bunker. In one timeline the Borg succeeded in assimilating all of Earth. In another timeline the ENT-E and her crew travelled back in time and thwarted the Borg. They tried to minimize the temporal incursions damage as much as they could but people died, Cochrane saw the ENT-E, Lily saw the Borg, and well as future tech onboard the ENT-E the crew of the ENT-E gave out massive amounts of spoilers to Cochrane about his future, and the future of Earth.
*it;s funny because Cochrane tried to run away from his destiny after hearing all the details the crew of the ENT-E told him. Riker had to shoot Cochrane and drag him in to the cockpit of the Phoenix just to ensure history would fold out the way it's recorded. You don't need the Borg to sabotage history. The ENT-E crew violating the temporal prime directive can destroy it all by themselves lol.
I digress. However the point i'm trying to make is that the Borg's incursion was not a predestination or casualty loop when compared to time travel mechanics in Terminator. The events in FC fit more in line with what we see in ENT with the Xindi arc, ST09 with Nero and Spock, TNG Yesterday's Enterprise, DS9's Children Of Time, and VOY's Timeless or Endgame. One timeline of events that was supplanted and erased by another timeline of events due to revision by use of time travel.