Two node cluster - some clarification needed

Hello,

I would like to clarify some things I think I know about two-node HA cluster.

1) If I understand correctly I can use quorum disk as a replacement for third node but that
disk still will be single point of failure? I mean if qdisk fails then whole cluster is doomed?

2) Qdisk could replace third node for a Proxmox cluster but will it do also for Ceph?

3) If I build three node cluster - will it possible to still have shared storage located on nodes or do I need
to use external storage?

Thank you!

PS. If it is answered somewhere else please point me to that place, I just found some clues
but not definite answers :)