Mera's Backstory in the Snyder Cut Contradicts DCEU Canon
After several years of waiting, Zack Snyder's Justice League and its 4-hour-long screen time allowed the film's writers and director Zack Snyder to give the characters more depth, a broader, more justifiable backstory, and greater individual significance. But, the film also gave room to the supporting characters, and writers used the longer run-time to give significant nods and tases to these characters' futures within the DC Extended Universe. Amber Heard's Mera was at a center point in this approach (besides Joe Morton's Silas Stone).
The character played an important part during Steppenwolf's attack on Atlantis' stronghold. She was even helpful in persuading Arthur Curry/Aquaman to join Batman and the team against the impending invasion. And her presence felt very connected to Aquaman and Atlantis's backstory while also being in accordance with her future outings within the franchise, including Aquaman (2018), which would've felt more familiar to Snyder's previous film if the Snyder Cut would've released back in 2017.
And as revealed in the Knightmare sequence during Epilogue: A Father Twice Over, Mera would be an important part of Justice League sequels as well. And it's visible from the response the Snyder Cut is getting, fans would definitely love to see her unite with Bruce Wayne's "new league", the Insurgency, to fight an evil Superman and "undo the world Bruce created by letting her die".
However, there is one important detail about Mera's backstory as revealed in Zack Snyder's Justice League, which is impossible to fit in the shared continuity of DCEU post the 2017 Justice League. It would have been easy to just forget about it if Mera had been a secondary character, but her mainstream role in Aquaman's storyline and an integral contribution to Zack Snyder's plans for Justice League II just totally make this loophole a cause of a massive confusion.
In 2017 theatrical cut of Joss Whedon's Justice League, Mera has a small conversation with Aquaman, where she discusses her parentage. She reveals how she knew Queen Atlanna (played by Nicole Kidman in Aquaman). She says,
"I knew her. When my parents fought in the wars, she took me in."
This implies that Mera's parents fought in wars between underwater kingdoms, when Queen Atlanna looked after her. In Aquaman, we see these kingdoms living in peace but are facing political differences due to King Orm/Ocean Master's (played by Patrick Wilson) constant efforts to gain support in his war plans against the surface. One of these kingdoms is Xebel, Nereus, who allies with Orm in his war campaign, only to later switch sides when Arthur returns with King Atlan's trident, commanding creatures of the sea.
Mera's backstory here makes basic sense. She was raised by Queen Atlanna, Arthur's mother. Her parents were fighting the Xebel Wars, a conflict between Atlantis and Xebel, when Atlanna took her in and trained her in combat. The war probably resulted in the death of Mera's mother (or probably she passed sometime after Xebel Wars and before Orm's reign; her death hasn't been discussed yet in DCEU canon). But, eventually the conflict ended with Nereus resuming his duties as King of Xebel, marking his debut appearance in Aquaman, which is set in the year 2018.
But, in Zack Snyder's Justice League, Mera's revelation of her parentage was different and completely knocks out Dolph Lundgren out of proportion and contradicts his appearance in Aquaman while also putting the future of Snyderverse amidst a bit of a continuity error issue.
Mera's Parentage and Backstory in Snyder Cut Contradicts DCEU
There is just a slight but a major change in Mera's dialogue when she's talking to Arthur after Steppenwolf takes away the Mother Box in Atlantis' stronghold. Instead of telling Arthur that her parents ``fought in the wars" when Atlanna took her in, she says,
"I knew her. My parents died in the wars. She took me in."
The fact that Zack Snyder's Justice League deems Nereus dead, despite him appearing in 2018's Aquaman contradicts the shared continuity of the DC Extended Universe. The contradiction further amplifies the continuity error as Jason Momoa has publicly vouched for the Snyder Cut being the true predecessor to his solo superhero outing directed by James Wan and has discarded Joss Whedon's version of Justice League. But, the Snyder cut completely discards a major character featured in Aquaman.
Nereus' Fate In Zack's Justice League Puts the Snyderverse Into Question
Fans are continuously advocating restoration of Snyder's original plans for DCEU. But, if they want Zack's Justice League to be canon with the ongoing slate of films planned for release and production in the franchise, this error will be a significant barrier in this dream's fulfillment. Dolph Lundgren's role in Aquaman was an important one. He had ample screen time and sequences to establish Nereus as a prominent character. Moreover, Aquaman 2 is in production and Dolph Lundgren may possibly reprise his role in the sequel. Even if he doesn't, you just can't possibly ignore his long presence in the first film and just imagine that Nereus is dead long before Aquaman ever takes place.
The reason Zack never changed the footage and his ideas for Mera's backstory in accordance with Aquaman in the Snyder Cut was that he intended his Justice League to be separate from the ongoing DCEU canon. In fact, Warner Bros. executives explicitly stated that Zack Snyder's Justice League was not part of DCEU; with Zack confirming later that since DC has went on with three films set after 2017 theatrical Justice League, he considers his take on the film to be set in a slightly alternate universe.
But now, after the positive response towards the Snyder Cut, Zack Snyder is willing to complete his envisioned DCEU. Though it has been explicitly announced that WB has no plans to greenlight any sequel to Zack's Justice League, the positive speculations around Snyderverse are just getting stronger day-by-day. And if it comes fruitful, Mera's backstory and Nereus's fate would require a fix.
The One Way to Rectify the Continuity Error
Most easy way to rectify the error in Nereus's fate would be to acknowledge that it was a continuity error. Marvel has done that earlier with its own cinematic universe, so just releasing a statement and then eventually neglecting this one small line in the future instalments of the franchise would serve as a perfect solution.
It's pretty much common for film franchises to make some retcon errors due to the involvement of different writers and directors helming separate projects. Sometimes, it's probable that writers and makers make small continuity errors while incorporating details from previous instalments in their fresh scripts. And for DCEU, these errors are more justifiable than other franchises as it has undergone tremendous amounts of changes, bad executive decisions, and abrupt changes in the studio's approach towards DC stories.
The future of Snyderverse currently hangs in balance as DC Films and Warner Bros. are focusing on the set slate of films awaiting release in DCEU, as well as Matt Reeves' The Batman. Moreover, DC is further focusing on planning some spin-off series for HBO Max, with John Cena-starrer Peacemaker being the first of such projects under preparation. Nevertheless, it would be better to correct Meta's backstory error as fans won't mind Dolph Lundgren returning to portray Nereus in future DCEU ventures.