michael -- you've really done your research! i think that maybe this was just a slip up. maybe they just wanted to have ice skating on the television to show the difference between jack's house (football) and monroe's house (skating), and ang wasn't too concerned with the actual date of the skating as it related to the movie. what's amazing is that you caught the name of the top defensive player!! who was it? and was there anything overheard from the tv when the skating was shown?
bookgirl -- that scene has me thinking too... it seems alma was kinda pissed off from the start. maybe because she was pregnant and possibly feeling irritable or under the weather? it seems that the comment ennis made about the girl's "wings" (as he tugs on jenny's ear) got her goat too -- as if the alma jr's and jenny's ears (maybe he felt they stuck out, unlike his?) were from alma's side of the family, and not his... after he said that, alma had an angry look on her face. then, they cut away to the scene of monroe hacking away at the turkey with the electric knife.
so, when she's in the kitchen, she's already seething, or so it seems. maybe she just had to work up the courage, and anger, to approach ennis. while alma starts it off with "me and the girls worry about you being alone so much..." she then starts in with her recounting of the tackle box note.. it seems that ang lee wanted to convey that she had been planning to say this to him, to finally let him know that she knew about his trysts. it was a delicate subject, something she'd been holding in for YEARS, and finally, it came out.
it's odd that she chose to do this on thanksgiving, with the girls and her new husband in the other room, but it sure made for a great, explosive scene.
to answer your question, yes, perhaps it solidified ennis' fears with this scene. it certainly set him off! but somehow i don't feel that without it ennis might have been more open to jack and their living together. my feeling is that ennis was so trapped in a cycle of fear that hardly anything would have changed his mind, except of course, the loss of jack. "you don't know what you've got till it's gone" type of thing...
i love how ennis says, about his bronc riding career, "...only about three seconds i was on that bronc, and the next thing i knew i was flyyyyin' through the air..." it's so cute how he says "flyin'."
ennis was effective at charming his children and maybe that's why alma was angry, because she felt ennis was trying to show up monroe, or something... maybe she was angry that ennis wasn't all he was cracked up to be, that she really did still love him, and would rather be with him than with monroe. ennis was, afterall, her first true love, and he was the father of her children. his secret ruined her marriage, her dreams were shattered, and their marriage vows fell to the wayside. her heart was broken. it's no wonder she spouted out "jack nasty!"
it's so sad, really... but the destruction of family life in the movie is excellently portrayed, showing viewers how homophobia can ruin the lives of men and also the people around them.