I’ve been reading Websleuths for years and finally made an account to share my thoughts.

(ETA: oh my goodness, I apologize for how long this got!)
I know it goes without saying that LE knows far more than we do, and that may explain why Ashlee hasn’t been arrested on the many charges people have suggested that have been used in other cases. What stands out to me is that she doesn’t appear to be repeatedly brought in or publicly pressured, which makes me think LE has a fairly solid understanding of what happened during the road trip and is working quietly behind the scenes. It’s possible they believe she didn’t harm Melodee, that someone else involved is their focus, or that taking her into custody could limit their ability to obtain more information about Melodee’s whereabouts from her or others. There’s also the possibility that public statements about her cooperation don’t fully reflect what’s happening privately. Whatever the reason, Ashlee doesn’t seem to be concerned with crafting a story for the media.
Regarding the recording Ashlee released: unless I’m mistaken, she specifically said the information better not end up in the media. From what we heard, she didn’t accuse him of working with LE.
That said, I don’t think the audio proves much either way. He asked to leave twice, and while his use of “yes/no, ma’am” is polite, I could see someone using it who feels threatened (such as a box cutter), trying to calm the situation. His voice does sound nervous IMO, and I was surprised that recording was presented as evidence against him. This again raises questions about why LE may be avoiding placing her in custody.
One other unrelated point that has stayed with me from earlier in the thread is a statement attributed to Ashlee’s mother, saying something like she strongly believes Ashlee left Melodee with someone who knew her father. I know that doesn’t mean anything, especially since I haven’t seen it mentioned in weeks, but it is still nagging me.
Whatever the case, I don’t think it’s related to the school, because why wouldn’t Ashlee just have had Melodee check in to keep the school off their back? Who knows, really. It’s all just so weird.
This is all MOO of course, and maybe me just trying to find any reason to believe Melodee is still out there somewhere. I agree with a lot of your possible scenarios as well, just wanted to offer some different type of thoughts I’ve also had.