Honestly, it's like Kleenex versus tissue paper. Maybe one is more on brand, but everyone knows exactly what you mean either way.
My intuition tells me that a direct message is a message directly to my username inbox, and a private message is basically the same thing but maybe encrypted or something.
The biggest issue for me is honestly the no life power mods, as you say. The stereotype of the pale, insulin insensitive, multiple mental health issue, living in their mom's basement, mountain dew enjoyer is there for a reason.
These people are basically the result of kids who grew up on 4chan /r9k/ and decided that they were going to have a job one day.
I have never heard of ETA meaning edit to add. I'm guessing FW means forward, I'm not sure if I've actually seen fwiw, and I think people just like IANAL because it has the word anal in it.
I agree with you on the til and eli5. I've seen these terms seeping out of Reddit as well.
One thing that made me honestly irrationally angry or somewhat disgusted is people saying happy cake day, and flopping their stupid emoji in there. My Reddit account was like 13 years old, and I never felt like I was missing anything there.