I know I tried to use an old sprint sim in my 3Xl once I upgraded to Barbet and TMobile said that the sim was still tied to the old device. It could be that you need a never-used sim if that is even a thing. I have figured out how to double-tap the back of the device to launch an app, you could google double tap back and then have that open a flashlight/routine of some sort maybe. I believe there is also a Magisk mod released for all google devices that changes some functions of the notification/time etc. Look around more and you'll find some of your answers either in the forum here or through google search.
2 and 3 are build into LineageOS. Don't know about shake to trigger flashlight.Solved!! H2O wireless problem. Got a new H2O starter SIM, the phone recognized it. Then I called H2O to swap my old number to the new number. Now I have both Fi (eSIM) and H2O SIM card working. If it did not work, the phone was going back.
Put the new SIM into the old phone it works there too.
Still looking for answers to items 2,3,4 from my first post.