This is another excellent post for a few reasons. First, I agree with your analysis on why the father was sitting, secondly, because it helps clear up this ‘dog was attached’ to the dad and illustrates the importance of sources. I’ll just go through a few things in order.I'm a couple of pages behind still but I strongly feel that we need to explore the term "attached" as it's used in reference to what most seem to assume is the dog being "attached" to the dad. I feel that this is a miscommunication, misinterpretation...
1. “KANGAROO BAG”. I believe this is false. The only source I’ve seen for this is from this article: Family is found dead on a trail without signs of violence. This looks like a news aggregator. So they’ve gone through the articles and amalgamated them. I think whoever wrote this saw this photo on the more reliable sources like WP and SF Chronicle and just made the assumption it was the same carrier (attached; apologies to the moderators as I cannot get a link to these articles but it is used on both as the headline photo). As Tower points out, though, this is just unlikely because a) the baby was much bigger, and b) carrying something like this on a hike of this kind would be very uncomfortable for both dad and baby. So, for me anyway, I am not outright discounting this, just putting it as from a low-value source.
2. “DOG ATTACHED”. I believe I’m the person you mention in relation to seeing this dog as especially bonded to Ellen. Of course all dogs are different but this was merely speculation on my part as at that point, I’d bought that the dog was tethered to Jonathan and so was extrapolating from there. HOWEVER, you are spot on in relation to sources.
The ONLY source I can find for “... attached” is the Mirror and Daily Star. That computes because both would be in a syndicate with an agreement to lift stories from each other. It looks as though whoever wrote this was sourcing from the bigger, more reliable outlets like the WP, SF Chronicle, and the Times. We can deduce this by comparing their reporting:
The Mirror: “... Mr Briese said the baby was in a backpack carrier with the dog near her father but attached to him...”. Note they don’t quote this. They’ve just rewritten it but in a way that is frustratingly badly phrased and misleading.
The Times: “... Briese said, Miju was 'in a backpack carrier near the dad, but not on the dad,'...”. You’ll note there that he is actually quoted. And I got this quote from the Daily Mail as the Times is behind a paywall (it is cited in the article, however).
So, clearly the Daily Mail is in a syndicate with the Times, SF Chronicle, etc. with lifting rights. This makes perfect sense as the Daily Mail can probably afford to pay syndicate rights. The Mirror and Daily Star etc. are essentially ‘cheaper’ tabloids so they can report news but they won’t have the same lifting rights. Hence, they have rewritten the story and included this badly phrased and misleading statement.
So, in conclusion, I am going to conclude (this is me personally of course) that there is no reliable source that the dog was tethered to Jonathan. This then immediately raises the prospect that the dog was already weakened and was being kept, along with little Miju, with the dad while mum went to get help. This is speculating and I still do think it’s possible the dog could have run off after Ellen but I think that’s less likely looking at all the available facts in the round.
Finally, therefore, I think you are spot on and hopefully this is something the investigation is considering. A number of people have been very interested in why the father was sitting upright and this provides the most likely explanation: he was positioned in such a way that the baby (and possibly the dog) were sheltered in what shade he could provide. That positioning then obviously gives us a likely very close time of day when he certainly was first fully incapacitated which is then further helpful for time of death and other important forensic information. So yeah, brilliant analysis. Apologies for snipping your post but I figured my reply was going to be long enough as it is.