Who's saying Prince was doing anything wrong as a Witness? His past behaviour has no bearing on the matter.
He was clearly not run of the mill but there is no evidence to suggest that the local congregation was wilfully ignoring behaviour that would get anyone else up in front of a JC. He was never going to be given privileges in the congregation but that does not mean the congregation is hypocritical in having a memorial service.
He seemingly attended enough meetings and shared his faith enough to be considered "in good standing". For goodness sake, I have not gone on the ministry in months, never answer, attend only 50% of the time, never fulfil any assignments let alone give any money - but I am still in good standing. I would be allowed a KH funeral if I died now.
It's very rare to have an invite only event but given the profile of the man I am not surprised, not least to stop the JW sycophants from flocking to the thing.