After you migrated your databases to your V4 instance, when you look at the “activation” tab of one of your devices. Does it show the OTAA keys? If not you likely missed the 4.7 redis → postgres device session key migration: [release] ChirpStack v4.7
After you do that on your V4 instance the keys should come back in the activation tab and the devices should come online.
I can’t answer many of your specific questions though as I have never done the migration myself.