We've got a million threads on this topic here with a few names of people that might have what you're looking for.
Try searching "tuning".
https://www.ecodieselram.com/forum/search/84038/?q=tuning&o=date
Welcome, a few topics around additives here.. and really most just use what they think is good for the truck. I've used a few, but I'm not sure if it helped or not over the years...
Welcome! If you really want that update.. taking it back to stock really is the only choice. Not sure it's worth it as most hate the update and it seems some are catching fire now (post update, which is weird...)
I don't know the exact figure. But yes, anything around that number (or higher) for a good distance will allow it to regen and get the soot level back to an acceptable amount.
Welcome, that "delete" word is getting hard to do lately. A few tuners out there that do the "deletes" since the crack down of the federal government on the tuning companies. We've got plenty of threads here pointing to a few people left that might do it.
If you have a spare stock ECM I would recommend using that during Service, as the dealer a lot times will take it upon themselves to flash in an "update".
Yes, as likely they are required by law now.. Everyone knows that if you bought something that was illegal for California and then that company got caught, why would a person be surprised they to are in the wrong?