Lately, I got also a couple of PMs from old and new accounts and half of them was a scam attempt. Of course always in such situation, the first thing which I do is to check these accounts and PM if it was for sure sent by the real account owner. Checked this account but everything was ok, active for a long time and with a good reputation. But additionally, I always respond to such PMs and ask if this is real and this time member responded back, that he never sent this PM and don't know what is going on. Hackers get into his account but never changed the password, only acted as the real owner.
I don't think you should make things complicated like that.
If someone go first and ask for financial supports or invitation to join groups, projects, and so on. I ignore such PMs all because they don't deserve my time to investigate who send those PMs, real users or hackers, and which intention they want. Such things always (or mostly) relate to bad purposes.
In contrast, if someone send me PMs to ask for help, such as ask for my permission to do translation from my threads, or ask for anything that I can help from my knowledge and skills, I will do reply and help them.
This is a real threat because everything looks fine, they delete PMs immediately after sending, login only when the real user is offline and if you are not lucky enough to get to the real owner with your PM, you can be scammed very easily if you fell into this great promotion or anything else, which is very hard to say no of course. I had to add that this was a high-rank and established member account, which is really scary and I think is worth to know that something like this already happened.
In serious cases, I think admins or global moderators can step in, but it is rarely without government/ police requirements, eg.
If it is personal issues between users, I don't think admin or global moderators will spend their time to do this, because basically it is not their responsibility.
Forum stores data on users' PMs 6 months after PMs deleted by senders and all recipients
Until the sender and all recipients delete the PM, plus about 6 months