For clarity the seat belt issue was experienced as follows:
I don't regard the following as "an issue" at all. It seems to me it is working as it should.
Join back of queue with seatbelt on.
Engage Parking Brake (because Autohold keeps the brake lights on which can annoy those behind).
First point. This is a scenario which Autohold is perfectly designed for. Indeed the availability of Autohold is why I can never be bothered manually enabling OPD ( One Pedal Driving) at the start of every journey and just leave the regen setting at 3, but I digress.
So you've arrived somewhere you want the car to be stopped until you press the accelerator to shuffle off until you need to come to a full stop again.
You slow down and then give the brake a light "double tap" activating Autohold and the brake lights come on as they should!
If anyone behind me is annoyed with the brake lights being on they're an idiot and can just get over it. Indeed, as the driver behind I want to see them on so I'm sure the car is not going to roll back on me if we're on a slight incline for example.
What you wouldn't do is manually engage the parking brake unless you really are going to be stopped for an extended period and aware that you will subsequently have to manually disengage it before driving off.
Queue moves.
Press throttle to move off - car bounces on suspension, parking brake stays on, car shows message to put on the seatbelt.
Well quite. So it should. You haven't manually disengaged the parking brake!
Granted, that message is misleading and a "problem" but hardly a significant issue.
Just turn the parking brake off before moving on.
If you want to be able to stop with the brakes on and be able to disengage them by pressing the accelerator use Autohold. That's what it's specifically designed to do.
Problem is cleared by released the seatbelt and re-applying.
Hopefully the solution is discovered before the cars queuing behind get too annoyed.
I think you've made a rod for your own back and I really don't think anyone behind would be the slightest bit annoyed that the car they're waiting behind has had its brake lights on.