Good thread. Will probably regurgitate what you said here a bit but you were quite thorough so good job.
Think around late teenage years it's easy to fall into this trap/behaviour sometimes out of the individuals own insecurities/obsessiveness and this applies to both parties within a relationship.
Most people just accept the relationship because they are scared of losing the person but while it's difficult to see beyond that, ignoring it is doing a disservice to both the perp and the victim.
I would also say it's important to note that the perp isn't defacto a horrible person and thus shouldn't be treated as such (depending on each individual case ofc). A lot of the time they're struggling with their own insecurities and that unfortunately manifests into toxic behaviours. Sometimes they grow out of it, sometimes they wont, but the worst thing you can do is reinforce or allow it.