You are just another one of many people complaining about the 8+microphone.
It seems we have all gone through **** trying to resolve the problem and have all spent hours on the phone with Apple second-tier senior advisers.
I am on my third 8+ phone. I also had no microphone issues with my six. They started immediately with the 8+.
You have done some really amazing tests and I don’t know whether your issue has been resolved or not but it doesn’t sound so.
Apple is very slow to admit that many people are now complaining about this microphone issue.
My carrier is Verizon and they replaced my Sim card. The advisor who I had spoken to many many days and God knows how many hours finally advised me it was a Sim card issue. He suggested the idiotic idea of exchanging what he thought was a compromise Sim card with another iPhone user having a different carrier. Why would I want to exchanged in the cars with another person and possibly infect their phone? What a ridiculous idea!
I went to Verizon and they whipped out an apple symbol card, installed it, changed a few settings which apparently affected microphone issues in my area which is New York.
I don’t know where you live but if it’s New York Pennsylvania Connecticut or New Jersey you might have a problem with one of your settings which should be on data only. Apparently any other setting caused static and poor voice quality in the microphone.
I don’t know if this helps but all I know is the deal I’m with Apple for five months and setting up three different phones, bringing them back to factory settings and putting everything in again was hair-raising.
Apple is far from user-friendly these days. I do not know what they are doing but I agree that they should not be putting our phones that may have hardware problems.