You probably can mount different NFS shares at each user's directory. For internal storage, you can't easily change the structure how ownCloud manages users. It used to be possible to mount external storage as the user's home folder in the enterprise edition but I don't find the reference again.
Your second request is probably difficult. Not sure how this can be done on home storage. You could just use the default ownCloud storage and connect to NFS via external storages and you manage permissions there in a way you want them to be.
If the enterprise subscription is an option for you, their support can probably help you best, which of your goals can be achieved. On the community version, you probably have to live with a work around or start implementing it yourself.