Thanks for this post. I've noticed that you did not even mention having an actual diagonal (y = 1x) as the reference line for a Q-Q plot. Is this really not a viable option - especially for (studentized) residual plots? Also: Did I understand correctly, that PROC MIXED plots=...panel; will use the first method you mentioned (=A line that connect the 25th and 75th percentiles of the data and reference distributions)?

P.S. I came across this post because I was confused how different the diagonals in PROC MIXED (SAS) and lmer (R) looked: https://i.imgur.com/7y2jOma.png