Modifiable snapshots are called clones in ZFS. Everything you said about snapshots is possible with ZFS with clones+snapshots. Clones are just as instant as snapshots.

Runnin ZFS in linux is not good idea. That's why I'm currently using OpenSolars and maybe switching to FreeBSD in the future for file and db servers.

The problem with btrfs time estimate is that it takes long time for filesystems to become reliable enough that you can put important data on them. ZFS has crossed that threshold, it takes years and years for btrfs.