- Yes, entirely possible.
- Yes, because he might've got the scroll.
- Where to start?
- How did @Tua put it? This'll be like one of these logic riddles. Which sounds fun, don't get me wrong, but is probably not the most efficient way of doing that.
- We don't actually know that Ami cares about Mari. For all we know, she hates her like Hana, but she's better at concealing it.
- Doing that would imply that our trust in Mari is now negative: we're so suspicious of her we're selling her secrets to a relative outsider, so that we may get said outsider's opinion on whether she's lying to us.
- How do we know Ami isn't in on it? They've had a long off-screen conversation; maybe Ami is fully on board with whatever Mari is doing, and will support her in favour of us.
It is one thing to give Ami a bullet she is never going to use to shoot us. It is another thing entirely to let her know that our most competent social agent is a shattered soul that is barely functional underneath the mask. This invites far too much in the way of subtle manipulations on her end.
Good points about the risks, although I don't think Ami would do anything to hurt Mari or the clan since they're important to Keiko.
The bind we're in is that Mari isn't a reliable source of information about her own problem and I'm not sure there's anyone we absolutely trust who would know what's going on. We could /try/ asking Kagome if he knows anything or could speculate about what effects TLitF might have on the user, but he's terrible at keeping secrets and it's not really his area so I think the chances he knows are low.
Noburi, Akane, and Keiko are trustworthy, and if we lay out our suspicions I think we can rely on them to do anything in their power to help, but barring getting really lucky with Keiko's inference engine I don't think any of them will know or be able to figure out what's going on on their own. That said, Keiko's inference engine is probably worth trying as a first resort. Maybe it can take all the pieces of evidence we've got and construct a model with them, then figure out what we should do about it.
At that point we're down to outside the clan options for information. Keiko could try the Nara libraries. I'll bet anything they have extensive notes on everything they know about the effects of various jutsu, in order to identify and defend against them if nothing else. Since we don't need instructions on how to actually perform the jutsu, that's probably relatively low-security information, and checking the library comes with the benefit of not having to share the secret with anyone.
Keiko could ask Shikamaru and he could use his resources to find out. But he'll absolutely figure out why we're asking even if we try to conceal it, because it's the Nara. I think he's more likely to help than try to use the information against us, but there is some risk. On the plus side if he decides to help he'll probably come up with a good plan.
We could ask Anko. She's friends with Mari, presumably cares about her wellbeing, and knows a fair amount about genjutsu. If she doesn't know the answer, she could probably find out without it getting out that we're the ones looking for the info. She could also be useful in convincing Mari to accept help.
As the highest risk option, if we get really desperate for specialized mental help we could ask Ino if she or someone in her clan would be willing and able to help a ninja with mental issues in strict secrecy, without using anything they learn against them or their clan. We could be somewhat deliberately sloppy in a way that implies that it's Kagome we're asking about instead of Mari. (Everyone knows Kagome isn't quite right, and he doesn't have any position of authority so it's OK if they think he's having problems if we don't go through with having them help.)
We would of course need to handle anything we did in such a way that Mari doesn't feel betrayed, which is another complicating factor.
Thoughts?