how many full nodes (non pruned) is it considered to be the minimum threshold for the network to remain safe from disappearing?
The network won't disappear as long as there is at least 1 node. The network will remain decentralized as long as nodes are run by independent individuals around the world (instead of in one jurisdiction).
The only problem I can think of with number of nodes is synchronization of new nodes. I don't think there is an exact number because it depends on both "supply and demand"!
For example if today all at once 1000 people decided to run 1000 nodes and sync from scratch, the network should be able to handle it since we have about 9600 listening nodes which those 1000 new nodes can connect to and download blocks. But if let's say 20,000 people decided to run 20,000 nodes all at once in one day the network can not handle it and things become very slow for those 20,000 nodes (not for the rest) trying to sync.
If all nodes went offline, and people offline backups of the blockchain at different times, how it would a consensus be reached to continue adding blocks be formed?
The same way we reach consensus right now. We all follow the valid chain with the most amount of work.