"but until you've been in this situation it's easy to sit back and tell someone what to do, not as easy when it's happening directly to you."
Except many posters on the two threads HAVE been in this situation, one with their own son that ended in violence! It only changed when she threatened real consequences with the police.
I really hope this new police contact gives you a strong way to move forward. That text message was chilling. She clearly wasn't being lighthearted.
For some perspective I asked both my teens (my daughter is 15 - same age as the girl, son 17) what they thought should happen. Both said that you should get the restraining order, get your son out of the picture somehow for a while and be strict. They believe you are giving mixed messages to your son by letting him carry on as he is and that it feels like you're actually ok with the relationship - and thus the threat - even though you're saying you're not. Both say that your son will thank you in the end, that he may hate you for a while but that you can't drive him into her arms as he's now so brainwashed he's already in her arms - you need to get him OUT. For them that means full on, being the parent, strict "that's it that's the end, she's ruining our lives, it's over." My daughter said that he may need and actually want that full on takeover. My daughter thinks that your son is the one whose mental health is going to suffer the most, possibly for a long time, and for that reason alone you need to get him out of the situation, as far away and for as long as possible. He needs to reset, like a post cult debriefing.
My son confessed that his first girlfriend started doing the text harassment and wanting him to stop seeing his friends. In his words "it was a massive red flag and I just ended it as soon as she started." Both talked about red flags: I didn't think they would even know the term and it actually made me realise I should have had a healthy relationships chat with them a looooong time ago.
I know it must seem impossible for you at the moment - I truly wish you well and can get this resolved. It probably has been a wake up call for many of us with teens.