I'm sorry to hear about your friend, and that you've had some bad experiences with recommendation systems recently, whether it be on YouTube, Twitter, or Facebook.
I also had a similar experience after a friend was killed in a home burglary, and my news feed started pushing all the latest updates on the case as well as similarly tragic news stories. For a long time, these recommendations were a painful and unwanted reminder of my friend's murder. Even worse was that the algorithm was definitely trying to push increasingly extreme and fear-mongering content about home invasions, all to try to get my clicks, and perhaps unintentionally radicalize me in the process.
As for why this type of thing happens, I think this part from your last point is pretty much right on the mark, "none of this requires any complex algos". All it takes is data of your browsing habits, the webpages you visit, the keywords you search, the webpages your friends and contacts visit, etc. There don't need to be any database fields explicitly encoding this information, which is why it's so worrying, such a system can push personalized traumatic content a global scale with precision, all from keywords and algorithms intended to maximize click revenue.
The other side of the equation is the content that gets recommended. I'm not sure who created the YouTube video that mocked your friend's death, but in my experience I was pushed content from typical local news outlets, then less reputable outrage content farms, and finally even more despicable sources, like white supremacist sites. That's something about this experience that really disturbed me. I've seen uncannily accurate product predictions before, but usually the 3rd party was relatively reasonable. I hadn't really ever been recommended radicalizing propaganda before, not to this degree. I've read articles about some of the operators of such sites, and they often don't even believe the content that they're pushing. They know that it triggers something in people who are vulnerable, and they exploit that for profit. That whole ecosystem just seems pure evil to me. At least the recommender systems work most of the time, usually just showing me stuff related to my hobbies, cute cats, etc.
Anyway, the combination of incomprehensible recommender systems and toxic content on the web is definitely not good for anyone's mental health. Hope you feel better in the future.