Right! Let's talk about capabilities! 

Corban Yaxley is primarily a combat guy. His family runs the regularly scheduled illegal duelling club; he's one of the few Slytherins who stayed signed up for NEWT Defense when they realized Weasley was teaching it and Sept reports with resigned appreciation that he mostly manages to engage with the class as a class without being a pointlessly oppositional dick about it. His parents made him sit through two years of Runes and Arithmancy because they were hoping he'd be able to spec for curse design but he failed both those exams with flying colors along with Transfig and isn't in any of the theory-heavy NEWT classes, he's just doing Defense and Charms and Creatures and Astronomy. 

Hendrick Rosier is basically the same guy as Corban except that he's worse at everything and he's a dick to everyone constantly about his resulting inferiority complex. (Well, actually he's worse at everything he's ever tried. If he lived in a kinder society he'd be a perfectly well-adjusted happy elementary school art teacher. But that is comprehensively outside his sense of what possible things could ever happen or be socially acceptable.) On the subject of certain specific spells, he actually got caught just this week trying to Confund the OWL Charms examiner into giving him a better grade; reportedly she told him that he was getting +5 points for casting the spell actually quite well and -10 points for being a blinding idiot. 

It'd be pretty surprising, but not impossible, for either Corban or Hendrick to have Obliviated anyone; it's a very difficult spell and aspiring Obliviators typically take a dedicated seminar. It runs every other year (open to students who scored E or higher on their Charms, Defense, and Transfig OWLs) and is famously painful if you're a 6th year with a late birthday. It's running this year, but the only Slytherin in it is Rabastan Lestrange, neither of the NEWT-age Buchanans signed up for it-- the older one, Andromeda's roommate, qualified, but she prefers to be on the Quidditch pitch for as many of her waking hours as possible (has a job offer in hand for the Magpies reserve and everything). 

Nelya's in that class though. Which you'd really think would make it difficult to get her with it. 

Could you Confund someone into Obliviating themself?

"...I can't imagine high odds of the target of such a thing doing a particularly good job but in principle I don't see why not." 

...Would it be contraindicated for Nelya to try reverse-Obliviating herself on the speculation that she did it?  If Rosier was just going for 'forget the past however-many hours'...

That said, she's tentatively demoting Yaxley - knowing how to make a very precisely explosive thing requires actually being able to understand what you're doing, and he has very conclusively demonstrated he doesn't by all accounts - and perhaps garnering some level of suspicion that Rabastan Lestrange, as the person who was actually learning to cast Obliviate, might have done it - but he's close enough to Narcissa that she doesn't think he'd try this.

"... that's an interesting question! I think she'd still need to know what she was trying to put back in more detail than nothing but... in principle a reverse Obliviation is not structurally isomorphic to a False Memory Charm, otherwise it'd be impossible without the caster having an eidetic memory... really the question is, I suppose, if you construct a theory of what she's forgotten which would be adequate if correct to gather the rest of the details, but it's in some important way not correct, what goes wrong... why did I choose on purpose to marry a conscience, that would be such an interesting experiment." Performatively dramatic insincere sigh, fondly patpat the Gryffindor. "... Probably she just wouldn't be able to do it but I recommend very strongly that you be really quite sure before you ask her to try, the next most likely mode of failure isn't likely but it's lethal." 

"...It's my least worst hypothesis.  But that does not imply that it is at all a good one.  I just don't have good enough data, and I'm not sure we can get it without taking some kind of risk, at this point.  ...Wait, couldn't we check her wand?  Or is that too far gone by now."

"Even if you'd done it thirty seconds after the explosion I'm not sure you'd have gotten much of use, unfortunately. It'd've just been the shield charm, most likely. Priori Incantatem has weird behavior sometimes-- wands have weird behavior sometimes, more like-- and I hear sometimes you can get the last few, but it's nothing to do with the skill of the caster. I asked Ollivander about that once and he said something unhinged like that cedar has better memory than walnut and acacia only remembers if it feels like it." 

"I mean, if a friend of mine's wand was insistent enough that she was its wizard to jump out of the pile and burrow into her sleeve, why can't wands have memories?  We should try it.  Can't hurt anything.  And if it's not about the skill of the caster..."

"I have to imagine that a wand wielded against its owner might feel the betrayal.  Which is probably too much anthropomorphization, but.  Wands are, indeed, weird."

"...And if the Obliviate was 'self-'inflicted, then it might have slipped past the wards!  I don't actually know how they work, but do you think it's possible?"

"It's definitely not my area of expertise either but I wouldn't be stunned." 

Does she want to try this now, or try other avenues of getting more information? 

She cannot immediately think of any other ways for her to get information that do not rather constitute worse risks to take in her current position.  Whoever this is has already tried murder before and she'd rather not actively put herself in their line of fire by visibly poking around at things.  The background noise is already bad enough.

So, not really, because what she has is barely a hypothesis right now, but also yes.

Nelya can be found in the library surrounded by anxious siblings, trying to build an adequate replacement thesis project. Professor Spellman's not that much of a stickler for inflexible policy, he's realistically going to give her a passing grade regardless given the situation (Professor Weasley did go alert his colleague that he's reasonably sure her project putting her in the hospital wing wasn't her fault), but her pride nevertheless demands of her that she turn in something. 

(Ted's not here; he turned in all his notes, promised Spellman he wouldn't touch Nelya's replacement project in hopes of reducing its odds of having politics happen to it, and has been doing his level best to barely exist to any but very determined attempts to track him down. This is not even that weird; it's nearly NEWTs time, and most of the 7th year classes are over but for a few project deadlines, and all of the 7th years are frantically studying, which means quite a lot of them are unfindable unless you have access to their bedroom, their favorite rooftop ledge, the random empty classroom they're assigning themself a study hall in, &c. Ted is a little more unfindable than that unless you are Andromeda, though.) 

Andromeda on the other hand has been very visibly spending all her time in the Slytherin common room, glowering at everyone.  

(If you are Ophelia Prince you might have noticed that some of the time that's not actually Andromeda, it's Cedrella with de-aging potion in her tea for a few hours a few times a week. Andromeda is building a house in [target country redacted] to flee to on graduation day and is going to owe her aunt Several favors for this.

Even if Ophelia wasn't able to detect the difference in their psychic presence, which she is, she might have noticed that every time this happens her notes on the Slytherin social scene expand slightly; Cedrella is actually trying pretty hard to follow Dumbledore's rule about not Interfering With Hogwarts Business but she's observing from a vantage point that is in principle available to Team Good Slytherin Students, i.e. Andromeda's chair in a public space, and not reporting anything that Andromeda couldn't in principle have noticed herself if she were the one doing it.)  

Anyway, here's Nelya! She's industriously working on a PURE MATH problem which HOPEFULLY it is not POSSIBLE to make EXPLODE. Unless maybe you are LITERALLY SPELLMAN HIMSELF but if that's who tried to kill her she is in any case DOOMED. 

"Hi Ophelia! What's up?" says Septima, who is fully unaware that Ophelia has been investigating at all. She asked and Karina said so. 

Oh, thank you, Madam Cedrella.

"The amount of time Andromeda Black has been looming menacingly in Slytherin's common room, after someone sabotaged Nelya's project.  Speaking of, I've a question that seems to require her particular experience, if she has a moment?  Though preferably sooner rather than later.  I don't want to run out of time to finish the project I'm working on; the relevant material's only going to be in the castle for so long."

It should be noted, at this point, that Ophelia Prince seems to have developed a habit of somehow muffling any and all conversations that seem to go above a certain volume in the library.  It's not even only conversations involving herself that she will mute.  It's any sufficiently loud ones.  (Nothing to see here!  Just someone who respects the library!)

(This is not an effect she could have derived from the Standard Book of Spells, as it happens.  But she has an unfortunate amount of experience keeping loud noises out to draw from.)