It sounds like you're answering your questions in your post.
Strengthen the pinkie if it's a concern. To push a key down, you've got the finger, the hand, forearm, upper arm/shoulder, and torso. Arches are good. Try just leaning forward from the torso and getting the keys to go down. You'll see where there fingers need to go for good support/arches.
Try scratching the keys with your pinkie (or other fingers) like you were trying to scratch the paint off. Extend farther than you would for practice, compared to actually playing the piano.
For the pics, straight/in line is good and more relaxed. If you have to, bend at the wrist a little. Or shirt in the seat or lean over.
Just from watching the video, other things....
Right hand. Finger tips. Try the scratching thing. It doesn't look like the first sections of your fingertips are coming in much.
In general, don't be afraid to be more aggressive. It might off/politically incorrect, but I've seen a lot of female musician focus more on sound and playing correct. That's the effect they generate -- sound is "nice" and it's all correct, but... who cares? Don't be afraid to hit/press/smash the keys more. The piano's not going to break. And if it does, it's fixable. I just got that careful-but-nice-sound musician vibe when I watched the video again.
- On that idea, experiment with weight -- Same thing, you've got fingers, hand, forearm, upper arm/should, and torso. If the piano (or use something else) has a lid, put the lid down, then put your hands in playing position, leaning forward, and use the piano lid to start pushing your body up. You've got that much power available. What I see in the video is someone kind of brushing their fingers along the keys. Somewhere I read it's the torso for the main power, and then metered down in stages down to the finger tips. Try setting your finger tips on the keys or lid and then make your hand leap up by moving your wrist (or scratch in with the wrist). Your missing all that power available. It's not just the left hand pinkie. In the video your pinkie is there, but it looks like pinkie is being played by as part of the forearm in the middle of the video. In the beginning, it looks more tensed up, functioning as part of the hand. I'd add more arch, relax the left hand, Alberti bass is more of a rotating wrist like twisting a door knob.
Even the language, "finger" "pressing" key. What about dropping the hand onto the keyboard? Springing off the keys? Scratching? Fingers as hammers? Leaning into the keys?