Well, i think your answer is very exaustive, honestly i'm in the middle between what you defined "legal" and "illegal" and i do not like either when touch something and it cannot stay together but... i'm also sure that the coupling which open this topic it is absolutelly not going to destroy, demage or compromise the bricks!
First due to the tolerance they have and second because in this assembly there is no step or dangerous gap. Obvioulsy it's what in mechanical is called hyperstatic system ... but what in LEGO isn't so???
However I'm 99% sure that this is just a bug, which i hope LEGO's team will solve soon
PS: The passion of the MOCs saved LEGO Group from its dark time in 90's, then i think LDD, a part its commercial purpose, should also be designed to give some freedom (in responsible way, of course!) to its users who at the end exchange, buy or sell LEGO bricks from official or unofficial channels!
However LEGO is still a wonderful invention which run me crazy everytime today as 30 yrs ago
Ciao