This is our current model workflow: DEV->UAT(deployed)->PROD(deployed)
I made a copy & archive of our PROD model. I changed the mode to standard and it is not deployed. I renamed this model DEV1. I made a copy & archive of our UAT model. I changed the mode to standard and it is currently not deployed. I renamed this UAT1. I made a revision tag in DEV1 and when i compare & sync UAT1 with DEV1 it says
You need not do a 'Copy and Archive' followed by 'Archive' to do a copy, you just need to do a 'Copy'.
You should change the UAT or Prod model to Deployed immediately after creating them, or else you might experience a break in compatibility between Dev and UAT/Prod.
For the main issue you're facing, can you do a simple change in Dev, then create a revision tag, and then try to do a Compare and Sync with UAT1. You can also get the message below if there's NOTHING to compare and sync.
Presumably, the Dev1 Data Hub will feed data into Dev1 T&Q, hence there's an import action in Dev1 T&Q that refers to Dev1 Data Hub.
There are 2 tabs in the actions page, i.e. Imports and Import Data Sources. Check the 'Production Data' in both tabs for the import action above. The 'Production Data' in 'Imports' allows you to run this action in Deployed Mode (for UAT1 T&Q and Prod1 T&Q). The 'Production Data' in 'Import Data Sources' allows you to modify the source model from Dev1 Hub to UAT1 Hub and Prod1 Hub.