Gosh! 
One thing at a time, eh?
Okay :)
Munchhausen's by proxy isn't even confirmed as a real disease - when it happens, it's really complex & weird. It wouldn't apply to hiding someone's antidepressants. It is, however, likely that Severus's partner is moving things around to "make her think she's going mad" - the classic Gaslight story.
Abusers love to do this sort of thing. Even if someone finds them out, they can pass it off as a practical joke (haha). Before I went through it, I used to enjoy practical jokes. Can't stand them now.
By sporadically hiding antidepressants, the abuser knows he will stop them working effectively - thus keeping their target on the back foot in two ways at once: neither properly medicated, nor knowing whether they've lost the plot. So it's important to get a handle on reality by CHECKING THE FACTS, and by TELLING SOMEONE in real life.
Calpol isn't essential medication. But it's important to the parent so, there again, hiding it will cause her distress as she can't calm the crying child. It's a control tactic.
With insults on the internet, I would advise blocking those sites and deleting all contact links with the abuser. Sure, he's doing it because he's getting a reaction from you. It hurts. But don't feed him. If you can't see the insults, you can't get wound up about them, so he won't get his satisfaction.
I agree with the stalking advice you found on YouTube. Get a £10 PAYG mobile, and have your number transferred to it. Make this a phone only for him.
Have your regular phone put onto a new number; tell only your closest people this number, make it clear they must get your permission to pass it on.
Never answer your old number on the cheap phone. In fact, put the phone on total silence. Check the texts once a day if you like, but never reply and never delete them (evidence).
The purpose of all this is to allow him to play out his weirdness, while you get on with your own life regardless.
Do exactly the same with your email. Set up a new one, use your new one for people you want to communicate with. Don't publish it anywhere. Set up a filter on your old email to "mark as read" everything that comes in from him. Again, you can check them every so often but never reply or delete.
Don't try to beat him. He's living out an extended rage at you. It's like a big, scary version of a toddler tantrum. The more you react, the more he rages.
What do you do with a tantrumming toddler? You put a safe distance, then ignore, right?
Same.