Hosts at Fox News privately ridiculed Donald Trump’s claims that the 2020 election was stolen while simultaneously peddling the same lies on air, according to court filings in a defamation lawsuit against the network.

Rightwing personalities Tucker Carlson, Sean Hannity and Laura Ingraham are among those named in the $1.6bn action brought by Dominion Voting Systems, the seller of electronic voting hardware and software that is suing Fox News and parent company Fox Corporation for maligning its reputation.

“He’s acting like an insane person,” Hannity allegedly wrote of Trump in the weeks following the election as the host continued to push the so-called “big lie” during his top-rated prime time show, aided by a succession of election deniers he had on as guests.

Even billionaire Fox owner Rupert Murdoch was dismissive of the former president’s false allegations, the filing alleges, calling them “really crazy stuff” in one memo to a Fox News executive, and criticizing Trump’s scattergun approach of pursuing lawsuits in numerous states to try to overturn his defeat.

It was “very hard to credibly claim foul everywhere”, Murdoch wrote, adding in another note that Trump’s obsession with trying to prove fraud was “terrible stuff damaging everybody”.

Meanwhile, Carlson, one of the network’s most prominent and controversial stars, was disdainful of Sidney Powell, a senior Trump attorney who repeatedly claimed Dominion’s machines flipped votes cast for Trump to Joe Biden.

“Sidney Powell is lying,” he wrote to a producer, the Dominion lawsuit alleges. He referred to Powell in a text as an “unguided missile” and “dangerous as hell”.

Trump, Carlson said, was a “demonic force” who was good at “destroying things. He’s the undisputed world champion of that. He could easily destroy us if we play it wrong.”

Fellow host Ingraham told Carlson that Powell was “a complete nut. No one will work with her. Ditto with Rudy,” referring to the former New York mayor and Trump supporter Rudy Giuliani.

Hannity, meanwhile, said in a deposition “that whole narrative that Sidney was pushing, I did not believe it for one second”, according to Dominion’s filing.

Other internal communications revealed that Fox News executives, hosts and researchers used phrases including “mind-blowingly nuts”, “totally off the rails” and “completely BS” to describe the false election theories they were publicly promoting.

All were included in a 192-page redacted summary judgment brief filed on Thursday at the Delaware superior court by Dominion’s attorneys. A trial is scheduled to begin in mid-April.

The company claims multiple Fox News employees deliberately amplified false claims that Dominion had changed votes in the 2020 election, and that Fox provided a platform for guests to make false and defamatory statements.

“From the top down, Fox knew ‘the Dominion stuff’ was ‘total BS’,” the brief states.

“Not a single Fox witness testified [in depositions] that they believe any of the allegations about Dominion are true. Indeed, Fox witness after Fox witness declined to assert the allegations’ truth or actually stated they do not believe them.”

The brief highlighted an 8 November 2020 interview on Maria Bartiromo’s show in which Powell insisted Dominion voting machines were used to engage in election fraud.

Bartiromo knew what Powell intended to say before the interview, according to the filing, in part because Powell had forwarded an email to her revealing her source came from a woman who got her information from “the wind”.

The Fox News executive responsible for Bartiromo’s show, David Clark, admitted in a deposition he “would not have allowed that claim to be aired” if he knew about the “crazy” theory from the email.

The filing also shows how Hannity and others were critical of their own network for its early call of Arizona for Biden on election night, which enraged Trump. Hannity texted Carlson and Ingraham that the call “destroyed a brand that took 25 years to build and the damage is incalculable”, while Carlson called it an “act of vandalism”.

Attorneys for the cable news station argued in a counterclaim that the lawsuit was an assault on the first amendment. They said Dominion had advanced “novel defamation theories” and was seeking a “staggering” damage figure aimed at generating headlines and chilling protected speech.

“Dominion brought this lawsuit to punish FNN [Fox News Network] for reporting on one of the biggest stories of the day – allegations by the sitting president of the United States and his surrogates that the 2020 election was affected by fraud,” the counterclaim states. “The very fact of those allegations was newsworthy.”

Fox responded to the new claims in a statement to ABC News. “There will be a lot of noise and confusion generated by Dominion and their opportunistic private equity owners, but the core of this case remains about freedom of the press and freedom of speech.”

Associated Press contributed to this report