Rare earths are not "rare" as in hard to find, but "rare" as in a low % of the ores. Many rare earth ores will yield at least 6 different rare earth elements. It is more a refining issue than a finding issue. You can basically look up any tin mine spoil, uranium mine spoil and get rare earths. The POD just needs to be the Japanese identify the need at least 6 years before the war. Now the butterflies of the strategic industrial material department can be very large.
Yes, but the mines in Inner Mongolia have a higher percentage so my understanding is that they take less effort to refine. OTOH I was not aware that tin mine tailings would be a good place to mine for rare earths...my understanding was that until the 1950s the majority were mined in South America after that mines in California and Colorado opened and were a major source until the Mongolian mines were opened in the early '90s. From what you are saying it is more a % yield that drives specific rare earth mines rather than needing to find a special mine.