I came up with an answer to the _second_ question (how do I do this from a new instance?).
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Again, this worked for me. What I want to be sure of is: does this really work?
I cannot definitively answer the question but it you can get the system to boot and the data looks good what I would do is pg_dump the result and then pg_restore is back into a clean cluster.
And I still want to know: can I do this without creating a new instance?
Someone considerably more informed than I would need to answer this.