I have not read through this whole long thread, but I thought I'd mention that a month or so ago I purchased a 2018 Regal sportback AWD. I have never owned a vehicle with auto stop and start before. I really dislike this feature. And it is hardly "seamless" upon restarting. But without saying anymore about all that, a couple of weeks ago I ordered and then installed the Gen II engine ASS system.
SMARTSTOPSTART Engine Stop Start or Auto Stop (ESS) Override for Buick Regal
zicmoto.com
It only took me about 10 minutes to install it and it seems to work fine. And no check engine light or anything weird. I love the way the engine now drops to a super smooth idle when stopping. Now, if I get behind a bunch of cars at the bank or fast food drive up, I can choose to manually turn the engine off to save gas, or not.