In the first paragraph, do you truly mean mourning sickness, or is that supposed to be morning sickness?
When I first started reading, I wondered briefly if this was going to be like a book I once read (for the life of me, I can't recall the title . . . it's in a box in the garage; one of my mother's books) about a scientist who invented a serum that would allow him to travel back in time mentally, though his physical body traipsed around in the present. There were a whole bunch of problems, naturally, and at the end, he was hit by a train while wandering in his trance.
That was COMPLETELY off topic. Before I get properly on topic, I haven't read "This Longing," as I'm sure you know, so I'm not familiar with Tiberius Grimm (he must be a favorite [original] character of yours, to meet the requirements of this challenge) or his history with McGonagall. Therefore, I'm not entirely sure if there are undercurrents of meaning I may be missing, but this does seem like a piece perfectly capable of standing on its own.
I'll address structure first. You do seem to like breaking up stories into segments and arranging the segments in an unexpected order, don't you? I don't notice a lot of writers doing it, which is probably because writing it presents logisitical challenges at times and also because it demands more from the readers. We have to pay attention and not just snooze and skim through it. (Your stories are easy and enjoyable to pay attention to, though. You always manage to grab me somehow.)
Back to structure. I think it works perfectly fine to have four different points of view in this piece, though if you hadn't broken the story up this way, it likely would have seemed strange. Naturally, Grimm's POV was very important, as was Minerva's. These POVs addressed the emotional undercurrents of the events. Moody's POV was used to describe the more technical aspects of the death . . . the fact-finding mission. Frank was included again to show emotionality--Moody's, this time. I am impressed by your usage of minor characters to more fully describe the major ones.
Characters: I loved McGonagall! She is one of my favorite characters, and I thought you did her justice. Your style of writing, which tends to feel a bit old-fashioned, suits her very well. I don't know much about Grimm, but I enjoyed him as a character. I believe I got a decent feel for what sort of person he is.
Plot: I don't have a problem with it. I enjoyed it. I found it very interesting to view the death period from so many angles. It reminded me a bit of a crime/police drama of sorts, with more emphasis on the drama.
Realism/Plausibility: Obviously, this isn't going to happen in the so-called real world. However, I thought the characters seemed very real, as did all the emotions they experienced. Everything your characters did, I can imagine without having to stretch and reach for it.
Excellent and thoroughly enjoyable.
Author's Response: The pun on "mourning sickness" fits too well, and I can't change it, even if it is a typo (which it actually is, but shush). It's just too perfect. XD
That book actually sounds pretty interesting - I had a somewhat similar idea in another story of mine, but it didn't have that same problem of the body remaining in the present time. Instead, the overuse of the time-travel potion caused one to fall into a coma or go mad. Maybe I've also come across that book somewhere in the distant past and it inspired that idea. ;)
The undercurrents are heavily restrained here in order to make the story stand more on its own (I hope). There were more things I was going to slip in, but they would have required too much exposition, and I wanted to keep things on the simpler side, especially with the timeline jumping around like it does.
The out-of-order story is something I experimented with for a time. I think it's out of my system now, but it's still fun to try and write because I have to try and keep everything in order too, putting together the pieces of the story in a way that doesn't entirely rely on linear time. I'm glad that it works to keep the reader's attention and makes it more of a challenge to read. It's the kind of story I would prefer to read myself. ;)
Frank was a random addition, so I'm very pleased that he added to the story. I wasn't sure if that part of Moody's narrative would work because I was switching away from him, but it was necessary in order to show more of his feelings. It couldn't be done from his perspective because he would do too much to block it out, rather like McGonagall does, and I wanted more feeling, even in that technical portion of the story. This part was more mystery-like, which I enjoyed doing, and it was definitely inspired by crime dramas, which can be so wonderfully deep and entrancing, and I wanted to capture some of that same feeling here.
The characters being realistic and understandable for the reader is mostly what I was looking for in regard to plausibility. I'm trying to make this an original story, and making the plot work in the real world isn't working at all. There has to be either a historical or fantasy/sci-fi element to it, and how that element will work is something I'm still trying to construct.
Anyway, thank you very much for your review. You're so detailed that it's really inspiring to read and respond to each of your reviews. :D