I’m not the game expert on this stuff, but I’ll do my best to answer the questions you’ve got.
Yes, in my hypothetical example, 6 was the higher of the two Attributes in question.
In your example, with Cunning 8 and Might 5, yes if the character defends with those two Attributes, then the character would take damage to their Injury stress track and would have a maximum of 8 on that track. If, however, the character had defended with Daring and Might, the character would be able to split the damage between the Fear and Injury stress tracks.
If the character suffers 5 points of damage in an attack, using the example of the Injury track with a value of 8, the character immediately suffers an Affliction and, as I understand it also takes the 5 points of damage as well. When they take 3 more points of damage to that stress track, they will suffer another Affliction on the same stress track.
As for the four hexagons associated with each Stress Track, every Affliction causes a -1 penalty to the character. As was explained to me, these are cumulative. If the character suffers 4 Afflictions of the same type, the character becomes incapacitated. They can’t take any actions or defend against an attack without spending a point of Luck. 5 or more Afflictions of the same type causes the character to black out. They can’t take any actions at all.
Anyway, there you go. Hope these help.