partitioning and subvol strategy with btrfs

If you don’t have specific needs, use btrfs as you would use another filesystem. Separating /home is a good practice.

Personally, on home servers my only subvolume is /etc, so I can make snapshots of the configurations. This can be automated with tools like snapper.

Usually, there is little interest of restoring just a previous version of /var, because of the need to also restore /lib/stuff. It is an all-or-nothing situation.

Snapshots of /home can be VERY big, so the disk size management is soon a problem. It can be done without any problem, but keep an eye on the remaining space. Also, as snapshots can only be made on the same disk, they are not a solution for backups in case of disk failure. Think of them as something for the situations like "oups, I deleted this file two hours ago, but I still need it".

Bonus 1 : none. In fact, btrfs was designed in order to simplify the stack mdadm+lvm+fs. So it is really better to avoid it.

Bonus 2 : No, but make a RAID 1! Simple and efficient, your data will love you :)

Ninja bonus : you really probably want to have a good look at the btrfs wiki.