Given the radicalization of college-age youths these days, I find the violence sadly plausible. It is a tonal shift from the previous issues, though.
I think we are meant to feel the connection between Gage and Steph, which is why her decision to push him towards Babs is a sign of maturity - she is actually making a sacrifice, not giving something up that didn't matter to her. It's supposed to be a sign of her maturity - she's done a lot of growing up, and I think it was a good step. Maybe it was also to setp Tim or no romance or something editorial, but it has a strong emotional effect.
The Gage/Reaper plot I think it was decent - if we'd gotten the information over several issues, or at the very least, not in just one page, we could have seen why he became a Reaper.
Sideways is indeed a character, but he was created in 2017-2018, as a result of Metal. He and Steph hung out in the recent Young Justice arc (14-15), but there's no connection here.
Steph's battle with gravity seems to mostly be "Gravity hates me, so I'm going to do my best to show that I'm not afraid of it."
I dunno why the thigh belt was empty, but it's a funny joke.