First one question:
Are you aware of the following (in the case of measuring 16 speakers in one single PRIR)?
When you are in the following menu:
You should only press ENTER the last time, so when you have just finished measuring the last 2 speakers and all the speakers in the display have the white line around them.
So the first 7 times that you are at this point (but without all speakers having the white line around them), you don't press ENTER but BACK instead. That way you go back one step in the menu without saving the (incomplete) PRIR.
(If you press ENTER after every 2 speakers being measured then every time a seperate PRIR is saved with only 2 speakers.)
[Edit: But if I am not mistaken the last 8 2-speaker PRIRs are still in the recycle buffer so you could copy them to the permanent internal storage and then still create a 16 channel listening room using 8 different PRIRs, or use the composite PRIR builder to create one 16 channel PRIR from them.]