Late reply... been away...
Not a lawyer, nor do I claim to be an expert in legal matters... but I believe one argument RP and NS could use against the letters DM wrote, they were illegal against the court "no contact" order. But to my knowledge, DM was not charged for breaching that court order. So the illegal contact via phone "wonderwall" song and the letters he wrote to CN, defense could argue against jury finding out about that illegal activity, DM was not charged, and it could prejudice the jury.
Note that I personally don't agree with that argument, and apparently, neither did Justice Goodman agree. Thankfully the letters were allowed as evidence.