Shoulder engagement: I used to let them hang in regular deadlifts
since they were relatively weak, I’m trying to avoid that esp. with
deficit DLs by resetting between reps but I have no idea if it’s
working.
Resetting between the lifts on a deadlift is an excellent strategy. The idea of a deadlift is to lift the weight from a dead-stop and resetting really works the muscles harder. Shoulder engagement: you want to aim to pull your shoulders into your back pockets and push your chest out. It looks to me like you're doing this just make sure to keep it tight.
Sometimes my knees snap back into lockout, nfi if that’s bad and where
it comes from. (My first guess was the bar being off-balance but I
can’t quite see it in these vids.)
Honestly, I think you just have more mobility in your knee that others do. I can't really push my knee back into that spot. One thing I can see is that you look like you're driving through your heels; you often rock back on lockout and your toes come up. If that's the case, try thinking about your foot like a tripod and focus on driving through the anchor points. It will help with stability (and possibly the knee-snap-back). Deadlift can be thought of as a leg press -- you're just pressing the earth away from the bar.
My lower back rounded a bit when finishing the last rep of the second
set. I believe a bit of back rounding is deemed okay in deadlifts, but
I’m not sure if that’s the case here.
A bit of back rounding is fine in the deadlift. A neutral back is more like a range and the larger idea is that whatever position you find works best for your leverage, you maintain that position.
General setup: I use one plate under each foot and a wideish but not
awkward grip, does this count as a deficit? (I did manage to do a set
from two plates once but I definitely felt the effect of starting from
a full hinge, and I’m worried this would cause me to round my back too
much in response.)
You want 1-2" deficit usually, so 1 plate under each foot is good as long as that makes a solid and safe lifting platform.
Your grip is really wide. This is the first thing I noticed when I saw the video. You're almost doing a snatch-grip deficit deadlift. You want the setup for a deficit deadlift to be the same as your regular deadlift: you're only extending the range of motion through the use of the platform (or plates-under-feet). I would personally try using a more narrow grip, if that's comfortable for you, as that will let you pull your shoulders back more.
What the heck are you supposed to do with your neck during bent over
exercises. Mine is way too back because bad posture, but I don’t know
the cue to avoid just staring into the mirror by default.
Neutral neck. As much as you can try not to look up into the mirror; this rounds your neck and just causes some soreness over time. I usually stare at the floor at some point in front of me.
You can place a tennis ball under your chin if you really need a hard cue. Usually people look up to check their form in the mirror. You can also try: set up form > check form in mirror > look back to neutral and perform lift > check lockout form at top. Just do the lifting portion with a neutral neck.
Overall, they look like good reps to me. I would narrow your grip and focus on pushing through the entire sole of your feet. It can be hard to think of using multiple muscles at the same time but remember that you need to drive with your quads, hips, and back together.
In agreement with @Frank, I think a form-check on your standard deadlift would be helpful. I'm not sure if your hips are too high or if that's just the deficit deadlift.
After thought, it looks like you're just doing a double overhand grip? You're coming up to the point where grip is going to start failing you. Your options are hook-grip, mixed-grip, or wrist straps. You might want to look into the pros/cons of all those options and learn earlier in your lifting career.
