Hey.
We are having the exact same questions/issues we are dealing with at this point.
To answer two of your questions. Appstacks are read only so you can also copy them from your shared storage (NFS/SAN,whatever) to your local storage and if needed copy it back. Also, if you have multiple shared storages just sync the files from 1 datastore to another. If your database fails or an appstack is deleted, just place it back to it's original location and do a sync in the appvolumes manager on the appstack tab. They will automatically be imported back.
Second. Writable volume is almost always detached from the machine if you have it assigned to users. Because the volume is only attached when user is logged in. I must say that this only applies if you log off the session after a session is disconnected (or left idle for to long).
We did investigate a lot of options (and I am still open to suggestions) but at this point are looking into syncing it between our datastores. We have 2 Tintri's right now and they are looking into syncing thin provisoned vmdk files from 1 SAN to another storage based. Thing is if you use any other option my guess is it will be bloated to a 10GB file (if you go for the default 10GB writable volume) and will consume quite some space.
In an ideal world VMware or Appvolumes would come up with a method to take a snapshot of the writable volume without attaching it to a VM (try to attach a fe thousands a night 
), keep copies of it as long as you want to keep it (think of Microsoft Volume Shadow Copy) and make sure to have a disater recovery option of syncing the data to a new datastore.