It would transpire that Milner, who is currently serving a life sentence with a minimum non-parole period of 17 years for the murder of second husband Phil Nisbet in 2009, had bought a cellphone and sent the threatening text messages to herself.
She was later charged with perverting the course of justice and jailed for two years and eight months - a period she was still serving when her murder trial commenced.
Mr Kearns, in the letter written by his lawyer Kerry Cook, says he would accept a "full and final settlement" of $20,000 from Christchurch Police for the "significant emotional stress and harm" that he endured through his wrongful imprisonment.
"How the police treated me was just plain wrong, and they need to pay up. It's as simple as that," Mr Kearns told NZME. News Service last night from his Christchurch home.
The letter also raises concerns by Mr Kearns that police also "failed to seriously investigate his allegations about his mother's involvement in Mr Nisbet's murder" on May 3, 2009.
Just days after his stepfather's death, Mr Kearns went to police with concerns that his mother was behind it.
But police concluded he had taken his own life.
A homicide investigation was only launched two years later after a coroner refused to find the death was suicide.
Mr Nisbet's sister, Lee-Anne Cartier earlier this year received an undisclosed cash settlement from police after she helped turn around their initial botched investigation through her own sleuth work.
In the High Court at Christchurch yesterday, Justice Christian Whata took five hours to consider Mr Kearns' claim for compensation from his mother.
The judge concluded that it was "difficult to think of a more clear cut case", describing Milner's conduct as "outrageous and calculated", and designed to put her son behind bars.
Justice Whata ordered her to pay Mr Kearns $45,000 in general damages for his hurt, humiliation, stress and financial loss, and $10,000 in exemplary damages for the malicious prosecution. He also ordered Milner to pay legal costs.
Afterwards, Mr Kearns said the judge's ruling had given him "some closure".
"It's good it's come finally, but it's been nearly six years now, so it was about time something good happened."
Asked if he thought his mum would pay up, he replied: "I'm not going to give here a choice either way, even if I have to sell the debt to a debt collection agency. She can't run anywhere."
Mr Kearns felt the incident had not had a long-term adverse affect on him, but accepted it was tough at the time.
"It messed with my head a bit, just not being believed. I was only young, turning 19 the first weekend I was in there..."
Now, he knows he will never speak to his mother again.
"I'll never look at her as my mum again. She's dead to me."
