I don't think the quotes from UT and FotR are irreconcilable. The Nazgûl keeping the Nine is what the Wise of the West may have supposed to be true (wasn't it Gandalf who said that?), whereas Sauron holding them seems to be an auctorial statement by Tolkien himself, who knew a little bit more of the matter than his characters.
Apart from that, Sauron himself had no problem wearing the One in spirit form when his body perished in the Downfall of Númenór - maybe he taught the Nazgûl the trick.
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Und aus dem Erebos kamen viele seelen herauf der abgeschiedenen toten.- Homer, Odyssey, Canto XI
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