Really interesting stuff terry.
I have a vision of you out in the woods. Just you, your speaker and REW.......
Anyway, I don't want to send you back out there, but my feeling is that it would have been a good idea to use Sonnies calibration file for the ECM8000.
When I bought my ECM8000, I did some searching on the internet regarding the accuracy.
I found that when an ECM8000 was professionally calibrated, its calibration file was very close between units. A comparison of various calibration files I found on the internet revealed this to be true. They all drop off slightly and slowly below about 40Hz (and is only down about 1.8dB at 20Hz). There is also a consistent hump above 10Khz (consistent with your measurement).
I decided that if I used Sonnies ECM8000 calibration file, that it would therefore be close enough for me and as you say, at that point any room FR effects would be greater than any microphone inaccuracy.
I would use the calibration file for the ECM8000 I have attached next time you're out in the woods.
Attached also is the graph of the file.
ecm8000.jpg ecm8000.cal
brucek