Sumo: I think your stance is too wide for your current hip mobility. The general recommendation is for your shin to be vertical when viewed from the side and the front (though viewed from the side is more important, from your last vid they were not), and I don't think you are meeting either in this vid. Keep playing with stance width.
The back niggle could be coming from some spine hyperextension at the top. You don't want to lean back, simply standing up with your glutes engaged is enough. Squeeze your butt.
Bench: Looks like you are over-flaring your elbows off the bottom. You do want to flare your elbows a bit to get a nice backward bar path (B to C below), but you want to do so in a way that keeps your elbows under the bar. Think about pushing backward while pushing up. Widening grip might help as you also want roughly vertical elbows at the bottom
There are things to improve when you set up, as you manage to wiggle around a lot. I think you overpress the bar on unrack which loses some of the tightness you get too. Have a browse of Nuckol's
bench guide and self-assess some things to improve, one thing at a time. I'd focus on driving your heels down more too.