For there to be no hard border within Ireland then NI has to remain in SM for goods.
That's part of the backstop, as is NI staying in the CU along with the rest of the UK.
For the backstop not to come into force once the transition finishes the whole of the UK needs to remain in the CU and the SM, else the SM for goods element of the backstop would be needed.
The UK as a whole cannot be in the SM without FOM.
CU alone won't remove the need for the backstop - the backstop will be required unless and until the whole UK is in the SM.
This is nothing to do with movement of people between NI and I.
For there to be no hard border within Ireland then NI has to remain in SM for goods.
That's part of the backstop, as is NI staying in the CU along with the rest of the UK.
For the backstop not to come into force once the transition finishes the whole of the UK needs to remain in the CU and the SM, else the SM for goods element of the backstop would be needed.
The UK as a whole cannot be in the SM without FOM.
CU alone won't remove the need for the backstop - the backstop will be required unless and until the whole UK is in the SM.
This is nothing to do with movement of people between NI and I.