I may be totally out of line to make this speculation. Regarding the choice of Heath's clips, perhaps it's because Ennis was still straight at that point in the film?
You know, I'm imagining it went more like this:
Scene: AMPAS persons, compiling "In memorian clips"A: OK, then we've only got Heath Ledger and we're good.
B: Yes. Guess there's no way we can avoid using a clip from Brokeback...?
A: No, that's the one he was nominated for and the film got some awards here and there, too, so...guess not.
B: Well, you able to suggest a clip? Have you seen it?
A: No,
of course I haven't. Uhm... have you?
B: Nope. Wouldn't be seen dead watching gay cowboys. What do you take me for?
A: Well...then there's no way around it. We'll have to sit down with the film to find a clip that isn't - you know - gross.
They pop the film in the player and start watching. 10 seconds into the film:B: Hey stop it right there! That's him, isn't it?
A: I think so. Not easy to see the way he's hiding his face, it's at a distance and all, but...yes. I think it is.
B: Well, that's great! Good shot, wouldn't you say? Probably the best there is in the whole film, I bet. Let's use that one, then.
A: Thank God. Great. I was afraid we would have to watch guys kissing and what not... Anyway, we're good.
B: We sure are. What a relief. Problem solved, case closed, and no need to focus another second on Brokeback Mountain.
Both: *whew*