I think ZFS does make a difference between non-zero and zero (actually compressible data), but you need compression enabled for that. The highly compressible data does not have a block allocated at all, being embedded in block pointer.
zpool upgrade -v
For home storage I'm using some ZFS pools (mirror for important data, striped for other) and export them through FTP, AFP, CIFS, WebDAV using containers (you can mount host folders inside a container).
You should put a question, to get an answer.
Anyway, the reason why you are not able to see them is because they are not mounted. They are not mounted because the folder structure is already there when zfs mount runs. Search for this, there were some solutions.
The recommended installation method for Kubernetes nodes is using VMs, therefore Proxmox KVM support is fine. If you really need an UI, you have kubernetes-dashboard (with heapster for graphs).
The best way to interact with Kubernetes is through programatic means (entities descriptors, CI push...
Nope. They are not. Async writes are buffered in memory and that is an OS thing, not controller. Maybe the controller driver signals to the kernel that can send all writes in sync or you mount your filesystems in sync, then you will get all your writes directly in controller cache.