Hi Northern Star and welcome!
I have little to add except, in my 50 years experience as a JW - kid, teenager, adult, MS, elder, ex elder, now just nominally in for family - here in the UK I have never come across any JW that would not go to the police for a witnessed murder.
Mind you it has never actually been put to the test around my experience.
I know I have heard of those that have covered up drink-driving offences, drug abuse and theft at the low to middling range of offences and those that have probably perjured themselves.
Certainly child protection issues are generally viewed as a congregational problem and not a police matter. Witnesses would typically ask an elder or close friend's view before going to the authorities.
When I was an elder I just went to the police about two child protection matters but told them (the Police) that I was "breaking ranks" and hoped they could proceed without divulging my involvement. I further assured the police that if it was absolutely necessary to come forward publically then I would do so.
The couple calling on you are:
Happy to count time and a bible study on their reports.
Even crazier than most JW's if they would not report murder.
Why not show them their own older publications about false prophecies?
May 22nd Awake 1969 about young people "facing the fact" that they will not fulfill a career in this old system" which will be gone in a few years.
1984 Watchtower about the generation that will not pass away.
That sort of thing.