Please understand; I'm no perfect saint, rather a (partially) reformed 'A-hole'...(But kinda cute, and never been arrested, lol). Point is, I have some limited wisdom to convey, to those ready for that.
When I was doing aggressive moves, such as OP demanding conversation, about this or that perception of 'offense', some friend started saying "chill out, better to not focus on neighbor...never goes well".
OP, you've so far, violated your neighbor's peace and quiet enjoyment, of his rental space. I mean, FOUR times, you went downstairs, for a 'talk', feeling like he refused to open door. That's literally getting into 'legal' and 'lawful' territory, in my layman's opinion. Now, you want a bunch of Engineers, (and a few nerds), to start giving legal advice, plus methods how to SPY on folks, all the while claiming to be 'scared'...but not enough to stop OP from going to his door, like FOUR times after he expressed a preference for you NOT doing that.
He could start a legal case AGAINST YOU...maybe.
In that case, your (supposedly) vicious downstairs neighbor would need a few 'monitors' on your victim ass. Then, OP, you'd get to 'talk' as much as the judge can stand, and let him or her decide, if your neighbor has valid grounds, for a restraining order...on you.
Like I said, you might be totally in the right, but looking to me like you actually already violating HIS rights, to peaceful enjoyment of his space.
Heck, do the floors creak a bit ? Even that can contribute, to this little, tiny, itty bitty maybe, you've got a clutch on.
What other WEB posts do you have going, now ?