You are correct: the rear latch of the door is the actual key latch; and the PSD pulls that in snugly as the last thing it does....
I believe the issue is the rear latch doesn't fully engage due to air pressure when all the other doors are closed/windows are up. There really isn't any way to get around it except slam it harder, and unless you know to look at the panel alignments from the outside, no way to tell if it is fully latched or just partially. Our Honda minivan has a motor in that rear latch, so once it senses the slider is mostly closed, it pulls it in. The part number for the latch on the PSD version is different from the manual ... maybe @gregoryx can comment on if the power door pulls it in (I'm guessing it does, because it is momentum that closes it on the manual door, and the power door slides slowly).
When partially latched I don't know if the door will slide open when driving (mine hasn't), but as far as I remember there is no indicator on the dash that the slider is open/ajar, though in the Passenger van the interior lights will come on/go off as you accelerate/decelerate and the door sensor triggers on and off.
And do not underestimate the door on an incline. Especially when pointed down hill, I wouldn't let a small child operate the door/go near it, in that case. In San Francisco I've struggled to get it pushed up and into the locked position (Passenger van with glass, so perhaps heavier than a sheet metal cargo van), and then when released it will instantly fall ... and I expect crush anything in the way. All it takes is a slight pull on the exterior handle to let it drop, and at least in my experience if opening it one handed, no time/no place to grab it to stop it from closing/slowing it down.
I'm back in line for a 2023 purely for the PSD after being Balanced Out after 9mo of waiting. I was completely satisfied with my 2020 Passenger Van except for the door (ordered it thinking a manual door would be no big deal, and regretted it within a few months), so I'll keep waiting.
Interestingly, the manual door on our Sprinter was a challenge much of the time (and they're larger and heavier than the Transit). We finally added a pull-lever to the BACK of the door to pull it in and that stopped the need to slam it. From the outside, we'd slide it closed most of the way then push on the back to latch. Total forehead slapper how much easier that was than slamming.
We added a mid-point stop thing on the Sprinter. A bit awkward to use it but useful. So nice to just tap the handle on the Transit and it stops at any point you prefer.