Cloud formation stuck in UPDATE_ROLLBACK_COMPLETE_CLEANUP_IN_PROGRESS

I understand that root stack is stuck with UPDATE_ROLLBACK_COMPLETE_CLEANUP_IN_PROGRESS, which is very common behavior in case of nested stack where some conflicts happen and one of the most common reason is, if updates were pushed directly to child stack.

Since you don't have AWS support, so I'd suggest you to wait for an hour and let cloudformation timeout, once it times out, you should be able to do "Continue Rollback"

Also, you can try this, if it helps(very less likely):

  aws cloudformation continue-update-rollback --stack-name <Root_Stack> --resources-to-skip <Child_Stack> --profile <CLI_Profile>

Please refer Cloudformation Update Stacks, specifically "Cancelling a stack update", "Prevent updates to stack resources", "Continue rolling back an update"

Please refer this re:Post

PS: AWS support is very much of help in such scenarios as they do take care of things from their end for faster resolution.