Lord Mountbatten being linked to Kincora, was a very successful , black propaganda  operation , carried out by the Provisional IRA. Instead of a massive media success for them, in the US, for killing him, his murder instead caused outrage.

Something had to be done and linking him to a Child abuse case in Northern Ireland, is what they came up with, some conspiracy theorists even , say MI5 killed Him, because he was going to talk about Kincora.

Since the outbreak of the terror campaign ,Mountbatten had stayed out of NI , specifically because He had a holiday home in the Republic.