Hello,
Quick answers for the first two:
Question 1) You would have to purchase Phantom for each environment. The per-seat model doesn't work across instances. Your sales engineering can help answer further questions on that .
Question 2) This is not something we support today but is a planned feature.
For question 3, there is a blog here: https://www.splunk.com/en_us/blog/conf-splunklive/introducing-the-splunk-phantom-platform-version-4-0.html (which is a little dated but still in the 4.x family) and documentation here: https://docs.splunk.com/Documentation/Phantom/4.8/Admin/Clustering
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