I wanted to address this, because I am one person who has spoken up about the assumptions around NG's abilities. Everything here, MOO of course. I am nowhere near NG's age (I am actually younger than SG), so this does not come from a place of defensiveness. Disagreement on matters of opinion does not imply bad faith. I think most of us here are doing our best to figure out what happened, based on our own personal experiences, which is about all we can do, given the lack of facts.
First, I completely respect your experience and knowledge as a lawyer. I can only speak for myself, but when I opine on the abilities of NG, I am neither making nor predicting any legal argument. We are so far away from this being in a courtroom that it seems pointless to speculate on, and I don't have the knowledge to do so anyway. I really don't think most of this discussion is about what would make for a sound legal argument at this point; it's about what the scant evidence we do have might suggest about what actually happened, with the goal of finding NG. This thread is about finding her, not building a case against any perp. All the legal stuff will play out later and is a separate question.
If we review all of the available information, I think we'll see that there are some conflicting perspectives on how mobile, active, and able NG is. My personal opinion (nothing more!) is that her frailty may be somewhat played up in the public communications to the ransom note writers and in the press conferences, to engender sympathy and humanize her. In these contexts, I do not think that it would help achieve anyone's goal to emphasize how strong and independent she is, and how she'll be just fine on her own. That's just not the narrative they want to be weaving at this point.
This is not to say that it's not clearly (IMO) true that she has mobility issues, and likely that she's on important medication, is injured easily, etc. But it's also not to say that she doesn't regularly move herself around town without assistance, drive a car, and generally lead an active life. These things can both be true.
The bottom line of my frustration with some of the discourse here is that it is framed as "because she is a frail old lady, it's simply impossible that she did X, and therefore the explanation must be Y." As outsiders, we do not know definitively what she can and can't do. We can talk all day about what we might consider reasonable for someone in a similar condition, but 1) we don't have a ton of details about what her condition actually is — just snippets of incomplete information, and 2) what might be reasonable for other people is not really conclusive evidence of what happened here.
I'm totally fine with "hey, given what we know about NG's age and condition, this scenario seems unlikely to me." I'm less ok with "no one in NG's age and condition could have done that, so obviously this is what happened." Maybe it seems like a subtle distinction, but I guess I'm just looking for some nuance, some acknowledgement that we're dealing with an individual human being rather than an abstract average statistic, and an understanding that we're all doing our best here with very limited information.
None of us truly know what happened, and whether we like it or not, the bits and pieces of information we have do not point in a single direction. It's not bad faith to share our own opinions based on the combination of life experience and the information we do have.