You are not the only one confused by the studies on PEth. The ambiguous findings that you note are basically where the research stands.
When PEth results it is detectable for an extended period. That much is agreed upon. The induction threshold is far less clear.
USDTL Lab estimates it requires approximately 200g cumulative consumption across a period.
Gnann, etal indicates ~50g per day across a two week period but also states a single binge could have the same result. That figure is not quantified.
Dr Greg Skipper, who recommends PEth as complementary to EtG states that it becomes positive after 7 drinks. An earlier study,
Aradottir (2006) indicated a daily intake of 48-102g daily for 18 days was required.
Zyeng (2011) also noted that sustained consumption was required. Another study, that I can't locate at the moment, clearly states that a single binge episode would not show PEth positive. I believe it was either Helander or Hoiseth but don't know for sure.
It is my opinion that the science on this biomarker is infirm and experimental and another example of foisting a test on the marketplace as established well in advance of it's proven efficacy.
Wish I could be more helpful but the information simply isn't there.
You're just above the supposed threshold of 7 drinks (8.5) so your size and/or metabolism may make the difference. There is a wide inter-individual variation in the results.
Who is doing the test? It is a pain to administer and has to be shipped frozen.