pressure derating for plastic/metal tapered thread connections

Fist I would use UPVC which has a higher temperature /strength rating, I would check the info from suppliers (on the web) to get the strength reduction at the temperature you are considering to the minimum published which I think is 25C above that it drops off to virually 0 at 100C
the specific answer to your query re thread depth is "probably" but cant speak from knowledge of the subject

Your assumptions are correct. The pressure derating comes from the cutting of the threads and the reduced wall thickness. The plastic pipe people always prefer to thread a male NPT plastic part into a female NPT metal part. It has more to do with expansion of plastic part vs. metal. The plastic male thread will expand "into" the metal female thread in a temperature rise, effectively providing a tighter seal. Check out the Spears website. They offer a female threaded fitting with a stainless reinforcing ring to provide additional pressure capabilities and prevent failure due to over tightening.