RDS cluster and instances status mismatch || Not Allowing deletion

It looks like there is an issue with your RDS cluster because when you stop the cluster all instances inside that cluster should also stop automatically. Well that is what happened in my case. So, I suggest you open a support case with AWS (link). Also note that you cannot delete a cluster that is on a stopped state https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/USER_DeleteCluster.html