I'm not going to try to persuade you that the editing was magnificent, or that you should have liked it. But I think there's a logic to it, in all of Roeg's films that employ it, and in this one in particular.
Because Roeg used it in other films, we can assume he had a good enough reason for it, even if in our view his attempt failed. I've never read anywhere why he used it, but if I had to venture a guess, I'd say it had something in common with cubism. The idea behind cubism is that when we look at an object we know, we have knowledge of the parts of the thing that we can't see at that moment. If I'm looking at the profile of someone's face, I know what's on the other side, even though I can't see it at the moment. That may seem silly, but the cubists were trying to recreate the sense we have of seeing all the sides of something in our minds. Imagine, for example, you're looking at the left side of Robert DeNiro's face, and you think, "This is the side that doesn't have the mole." Your knowledge of what's on the other side affects your viewing at that moment. (Silly example, I know, but this is a film forum, after all.)
So the cubists tried to show more than one side at the same time.
Maybe Roeg was trying to do something like that, but in a more fluid medium: film. It obviously doesn't work so well when you're watching something the first time, but it can be quite powerful during a repeated viewing. Some of his brief cuts to other scenes can be jarring because they remind you of something that will happen later. And that's saying nothing about why a particular scene might be thematically significant at a given moment and not at another.
When I first saw "Bad Timing" years ago, I was really impressed with it. When I saw it again recently, I didn't like it. There is something pretentious about Roeg's work. But it works well in "Don't Look Now".
Sutherland's character is given a glimpse of a future scene when he's at home, a warning that his daughter is going to drown, but he does nothing. He has a gift which he refuses to recognise, and in fact he ridicules it in others (the two women in Venice). Throughout the film he is given glimpses of scenes from the future, always as warnings, such as his own funeral. The jarring editing here is actually part of the story. I haven't seen the film for years, but I remember thinking that the father's demise comes as punishment for refusing this gift, and for wanting proof (of the identity of the girl in the red raincoat).
While "Bad Timing" probably has more reputation than substance, as you say, I think "Don't Look Back" is a whole other case. And that's because the style and substance are working together nicely.
Very interesting ideas. One thing I think I noticed, the photo of the person in the red hood in the church we all assumed was their daughter. But her coat was patent leather. Much later in the film, we see the photograph again, and it's clearly made of cloth - the same as the "old woman's." It's possible that it wasn't his daughter's death he was seeing (and which he thought he was seeing), it was his own.