Pliny wrote in AD 112, two generations after Jesus, about one province. Doesn’t that show the worship existed early, not that it goes back to the apostles?

Correct, and Pliny should be read as a late data point rather than an early one. His job in this argument is to be an outside witness that worship of Christ was established and normal by the early second century, which already puts it two hundred years before the Council of Nicaea. The earlier evidence is internal: Paul’s letters from the 50s, the hymn he quotes in Philippians 2, and the Aramaic prayer Maranatha, all of which predate Pliny by half a century or more.