they appear to be incapable of holding a normal two way conversation.
This is totally normal for someone with ASD. It's one of the many reasons it's considered a disability.
But this networking issue is one that they need to learn and manage
Statements like that is why people are calling you ableist. That statement is the same as saying a blind person has to learn to see in order to work at your company.
Now if the job was "driver" the blind person's disability makes them unsuitable for the job. But this job isn't "networking" there's other aspects, so it's not the same situation for the employee with ASD, otherwise the statement "I'm probably autistic" would have had them suspended on medical grounds whilst it was investigated.
When it's time to leave an event you could politely tell them this. Or tell them in advance that the event ends at X time which is when they should leave and anyone else who is still there after that is staff who's job is tidying up. They're not leaving because they don't realise they're supposed to. Helping them realise should fix that, unless there's some other reason they're not leaving. So if it keeps happening, try asking why they don't leave when everyone else does. If you can find the problem you might be able to find a solution.
If you're present at their conversations where they're monologuing eg it's a group conversation, you could agree with them in advance a particular phrase or visual cue, designed to let them know they're monologuing but without being humiliating, therefore giving them the chance to stop talking or ask a question. This presumes they have some ability to not monologue though and are simply unaware in the moment that they're doing it, which might not be the case. It's also possible it's happening from stress and even if aware of it they can't stop.
The example with "Joan and the event" is them making an effort. Most likely having been told at some point, or picked up for themselves, that to abruptly ask the question you want to ask with no small talk first is considered rude/not the done thing. So they're trying to do something they can't do, make small talk. Only they've got it a bit wrong. On the plus side, they seem to be able to recognise that when someone walks off the conversation has ended, it could be worse. So they do have some social skills and can maybe learn others, with help.