1) The duplication entry is annoying but not an issue. It is a warning that while pulling information from the vSphere Database, a duplicate Datastore was found. In this case, the vSphere Database contains two Datastores called CMODE_Servers. The vc.data_store table uses the Datastore Name as the Primary Key. From the error message we see that vSphere believes that these are two different Datastores and NFS Storage (Notice the different free_capacity_mb - One has 173907 and the other 173908). It is possible that if these are suppose to be the same Datastore that the Datastores are mounted via different IPs. Regardless, it is a warning about the vSphere Database containing duplicate information. If I recall correctly, WFA will ignore the second entry and not add it to the WFA database.
2) What type of OS is DFM installed? If this is Windows, then you need to install an SSH Server on the DFM host. If this is a Linux install, then double check that the account you are using can SSH. It looks like the Certified commands are designed to use SSH. If you do not want an SSH Server installed on your DFM Server, then you would have to re-created the commands using API calls from the NetApp Manageability SDK. If you go that route, then you can add the DFM credentials to WFA using the Type of Other.
I hope this helps!