Questions regarding blocksize and write amplification

[Eizock]

Hello,

I have been running two different setups of ZFS Pools with Proxmox and VMs on them.

1. Raid-z2 (6 disks, ashift=12, volblocksize=64k) <-- Virtualdisk (reports blocksize as 512 in VM) <-- NTFS/Ext4 (blocksize 4k)
2. Mirror (6 disks, ashift=12, volblocksize=8k (default)) <-- Virtualdisk (reports blocksize as 512 in VM) <-- NTFS/Ext4 (blocksize 4k)

After looking in the Forums I only found some inconclusive Information:

1. ZFS size / allocated difference?
2. Improve write amplification?
3. ashift, volblocksize, clustersize, blocksize

Then searching in the Internet I found some more helpfull writedowns:

1. ZFS RAIDZ stripe width, or: How I Learned to Stop Worrying and Love RAIDZ
2. RAID-Z parity cost
3. Please help me understand ZFS space usage

I was able to answer most of my questions, but a few are remaining:

1. Why does the virtual disk always report as 512b blocksize?
2. Does this not matter because of some trickery in VirtIO/KVM?
3. Why is the Virtual drive not using the set blocksize of the storage its on?
4. Why is there no setting on a per VM/per virtual drive basis for blocksize?
5. Why has 8k volblocksize been choosen as a default for Proxmox (potentially risking a +200% space usage on ZFS, according to RAID-Z parity cost)?
6. Is there anywhere a good guideline for tuning the volblocksize of ZFS on Proxmox?
7. Can i change the blocksize of a ext4 filesystem to 64k or is that considered unstable?
8. Is the parity/padding/space waste on Mirrors nonexistant?

Some more observations (with more questions ) :

1. On setup 1) I have a (Windows) VM that reports 5.41T used, which should be around 4.92TiB, but ZFS reports 5.39TiB. So almost half a TiB more than it should. Discard is enabled. Is it because there are 3 different blocksizes involved?
2. On setup 2) there is a (Windows) VM, where when I write ~5GiB (sequentially) zfs reports writes of around ~15GiB so a writeamplification of 3. Eventhough there is no report of writeamplification problems on mirrors. (Atleast not that i could find.) Is it again because of the 3 different blocksizes?

Maybe I am being a idiot and missing something obvious. But any hints/ideas/links/pointers are welcome.
Cheers