I think you hit the nail on the head here. The word "conventionality" or the ideal of it is at the root of why she bore all this for so long, even though the knowledge of the truth must have undermined her sense of pride and dignity as wife and mother.
Ennis confuses me in this regard also. Perhaps initially he was willing to play the game? But something may have turned him off over the years. It seems like his sense of fatherhood withstood the test of time, even as the marriage itself deteriorated.
One can ask the corollary question as to whether he suspected that his children knew, but that is another subject (I think this was discussed in the past in the case of Jr.)
Well,I've always thought that Ennis loved and,even,could be in love with Alma in the beginning of their relationship-really,in love in "Ennis style"...-; he has found a woman who could give him love and cares,two things that he had always been looking for.And at the same time,he would had a social respectability,making what a man is supposed to do;good enough for his tendencies -very hidden until Jack's appearance-that for me have been provoked by his father's teaching about how bad they were-I´ve spoken about this in another thread...-.
Of course,mainly after his discovery in the mountain,playing the game for him was ,how do you say that?,"killing two birds with one stone",isn't it'.So,I think,yes ,he was willing to play in despite of his real feelings and HIMSELF.Moreover,as you say very well,his duties as a father were at the end fairly much before his husband's duties; and also were an easy excuse for not comitting him with Jack's wishes and,so,not being obliged to play "another game" he wished,but he was not prepared to.