Regarding whether it was a big fight that went wrong, or was more pre-meditated, or both......
Firstly, I dont see Allison as being confrontational to the degree of heading into a big verbal argument.
My hunch is that, as GBC was so controlling, Allison would have been pretty accustomed to tiptoeing on eggshells.
I doubt she would have engaged herself in a full-blown argument with him- (but I'm glad she eventually ripped into him with her nails though!!!!) due to the fact that she would have known from past experience that this was not an effective way of dealing with him. He would have always come out on top, and I'm sure she had experienced threatening enough behaviour before to warn her off flexing her muscles with him. Hence the usage of a counsellor to help them sort through their issues. I believe she could have been quite powerless and without voice in the marriage.
(I am even more convinced of this after reading about the signal system. No-one crossed the master, is what I'm guessing.)
Women in these situations know to tread very carefully, for the wrath of the master is not worth getting to have your say.
IMO the only way Allison would have ever been able to leave GBC would have been to flee, so I cant see her verbally threatening to leave him. Could be wrong. (reading between the lines with limited info...as we all are )
Also, someone suffering anxiety and depression would in my mind be adverse to full-blown arguments, as it only makes you feel more anxious and upset.
It would be my guess that she'd easily give up on going to the conference and go to bed with stress-induced paralysis. Afterall, she was not very present in the business really, and her main priority was functioning as a mother to the girls, so whatever that took is what she would give. MOO again.
I see the murder as pre-meditated and not as a result of a conflict.
The fact that he had not thought of a cohesive, rational story was IMO probably due to the fact that he expected to be believed at face value, and he didnt bother to think any further on this point, plus, the scratches were not part of the plan.
The thing I cant work out is how was he going to claim the insurance without a body.
Maybe that part of his plan was yet to come??????????????