For question (1), I wanted to add further, with zero nodes, the idx has to be managed explicitly, so if the zero2 with idx=2 was /removeNode, you would add a new zero with idx=4.
For zero wal and alpha postings and wal, persistence is needed to have recoverability. The state of the groups will need to be maintained, such as membership, timestamps, etc. and you losing such state is destructive. For alphas, obviously the db data, so if a node is lost, and db data is not persisted, db data will have to be restored/replicated from other alpha nodes.
So I recommend having a disk that can be re-attached. Thus if you had alpha1 that is lost and replaced, the external disk for alpha1 can be reattached. If alpha2 was lost/replaced, you’d need to reattach external disk for alpha2, and so forth for all nodes.