It makes sense that if someone has replied to your PM you should be able to send them 1 reply, then another if they respond again. Decent way to carry on a private conversation. I'm not sure how easy that is to implement here though to satisfy those conditions. Unsolicited there is not much that people can do, but if they are reaching out to you or posting in your thread first, it might make sense to carry on all conversation there, and then finalize anything of a personal or privacy focused nature into a PM from their side, that way it should be reasonable to fit into 1.
if they are going to start their conversation outside of this forum then they should contact only if the bitcointalk account has got that information on their profile.
Should be replaced with currently using apps like Telegram...
Yeah there is nothing you can do if they just pretend to be someone on another platform from the forum to stop that. I've had people contact me to ask if it was me messaging them , always glad to point out it isn't and good on them.
There was a discussion a while ago about adding in new, chat platforms. There were suggestions given to theymos not sure if he plans to do anything with them. Some people just type in the
"Telegram - [username]" in say the icq field.