You should stop both Logstash and Kibana, and start them after the cluster is up again.
Not sure, I do not use Elasticsearch on k8s, so I do not know what will happen if you scale it to 0. You just need to stop the pods, increase the space and start them again.
No idea if this is correct, as I only use Elastic on traditional VMs and I do not know how this shared persistent volume works, but you need to make sure that you won't hit the watermarks, since the underlying storage is shared, all your cluster will probably hit the watermark as the same time, and this can be problematic.