I don’t think that length of time makes a difference as to whether someone trolls or not,in fact it gives them more credibility meaning that a longstanding poster who trolls under a different name but on the same account is far more likely to be believed if they write “namechanged,” and when mn hq check they’re tied to a longstanding name.
But one thing which absolutely would make a difference IMO is the ability to namechange without limit. There is absolutely no reason why posters should need unlimited namechanges, no other forums allow it,and although other forums do attract trolls I would say that MN is by far the worst.
Also, taats being deleted just causes more taats to appear.If the discussion could just be had on a thread at the time the troll is shown to be a (suspected) troll, then the thread would probably run to its limit and be virtual chip paper by tomorrow. As it is I’ve already seen three taats this morning on the same subject.
Truth is that posters are banned for all manner of reasons including troll hunting,so pbp doesn’t automatically equal was previously a troll,and given this is an internet forum where posters all post under usernames it is literally not possible to know if a poster is a troll or not, but there are IMO certain things which should set alarm bells ringing. I.e.poster posting from the ambulance while on their way to hospital having never posted under that name before. Poster posting in the heat of the drama i.e. “I’m hiding in the toilet and can hear dh trying to smash down the door with a hammer, he’ll be in in a minute, OMG he’s got the axe with him, am sooooooo scared.” Or “I’m lying on the floor in a pool of blood,so scared I’m going to lose my baby,”with pictures once baby arrives into scbu and posts of how it’s touch and go. Or posts where the mother/sister/husband comes on to update the thread. I can just imagine my mum if I asked her to update mn on my behalf. 
in fact when I was seriously ill in ICU a few years ago she pre-emptively warned dp not to post about it on facebook, which he didn’t but still....
But with that in mind, I do think that the over supporters looking to be the most supportive ever are partly responsible for the trolling problem which exists on MN. If people just took a step back and thought that there’s no way certain threads can be real,and didn’t constantly “check in”/ask for updates/talk about how they’ve been up all night crying thinking about the OP and to make sure she knows they’re here to talk, the trolls would be less encouraged to troll.
As for posters talking about it being mn’s site to do with as they want, absolutely it is. But reality is that what appears on a site is what gives it its reputation. Do MN really want that reputation to be that most of what’s posted on there is bollocks but they let it stand anyway?