Hi,
1. The witness must be on the same subnet as the active and passive node?
Yes because they have to communicate in order to sync. Check this article: vCenter HA Deployment Options
2. If it doesn't have to be how much better it is in the same subnet or another? Check the previous article.
You need a subnet for the nodes (an internal subnet) and then an additional interface for the active node.
3. What will happen if the latency is not guaranteed between all nodes? Failures can occur and they won't be on sync.
4. What if the witness cannot be reached? The VCHA could be broken if there is a lot of time since they have been on sync.
Hope this helps.
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"I greet each challenge with expectation"