We only have the published story to go on, but assuming she was correct, her tourist visa was valid... there's was no justification to lock her up- a 60 yr old woman with zero crimes, not even a parking citation- for 6 weeks because what?

I would assume that the paper she signed contained a bunch of admissions that she had violated immigration law. Again, we don't know for sure. But that's what one would expect - in order to get the "deal" of voluntary self-deportation, you're going to have to make an acknowledgment of guilt. Once she signed that, she was basically toast, in terms of any rights that she might formerly have had. If you confess, you're going to get the consequences of a guilty person - even if you haven't done anything, not even a parking citation.

I hate to say it, but the six weeks she was stuck in detention is probably explained by Hanlon's law. "Don't attribute to malice what can be explained by incompetence." ICE agents might get a bonus for the initial arresting of or detaining of someone, but it's not likely that bonus is at all affected by how long they stay in detention. There probably wasn't anyone who benefited at all from her staying for six weeks, rather than a few days. The most likely explanation is either that the shutdown threw ICE off, or they just never bothered to do anything with her from some other sheer bureaucratic stupidity. I've dealt with bureaucracies in my practice for decades, and many of the most frustrating things come not from someone making an affirmative choice to do a bad thing, but from people just not taking action at all if it a choice isn't being forced on them.

Again, the real problem was likely that she signed the papers they gave her. They sold that to her as making things simpler for her, but in reality it probably made things much simpler for the government - which meant that her case wasn't something that required anybody's attention. Which meant no one was paying attention, especially during the shutdown when you would expect a lot of normal processes/personnel that might have kept a case moving forward would have been temporarily unavailable.