Let me start by saying please let support and your vmware rep know about this. We have been pushing for over 2 years that the media model needs correcting.
That all said 
1. Media needs to exist in the provider VDC that it will be used, so if an org has 3 VDC's against different providers - they would need 3 copies. (Even if the media is on a datastore that all the providers could see just fine, ie NFS)
We have had to take the stance, sorry - we can not support vCloud ISO media as the feature does not work in vCloud with our user base (30k + vm's) - for these users they have to use the VRMC, normally they use a local jumpbox and then mount that way so network speed is not an issue. (But this really is a pain)
2. VRMC media mount really is for letting a user mount a local iso/drive - something that only works well in the same metro network.