2nd question
Inside the task sequence, there is an "Join Domain or Workgroup" option where you can have the device join a domain. I have never used it separately from imaging but I don't see why it wouldn't work for what you want to do. I would try to have it perform the backup, restore, then add to the new domain.
keep in mind you have to have an account on the new domain so SCCM will have rights to add the device.