I really think you need to contact via PM our member PSE
He has been battling the NHBC with a similar defect.
It also, (reading between the lines) seems to be similar concerns in your mind!
Like you, he has Barry Hestletine MBE (87) as the NHBC's "independent" expert.
So the NHBC claims manager tells you the NHBC use him a lot!
You do have to question just how independent he is - practically an employee?.
It also goes some way to explain why he takes many weeks to produce what is by that time, a relatively straightforward report based on many previous test results from many different test labs.
It also demonstrates that the issue of weak or sub standard mortar is widespread and something that industry needs to address by regular testing of mortar cubes taken on site each week.