I guess "in extremis" is valid if you speak Latin, or know what
it means, but I'm not so sure it's part of common usage. Anyone
has the right to use it, but it seems a tad artificial, not that it
makes a difference.
BTW, what does in extremis mean? I've never heard it. If you
do decide to answer that, please do it privately. Though I doubt
enough of us know it to spam the FFML (or maybe I've read
different things than everyone).
Actually, as Latin phrases go it's not that uncommon. It means
pretty much exactly what it sounds like: "in extremity" (or, as you
suggested earlier, "in these most extreme circumstances").
Such phrases are ultimately, I suppose, a matter of author preference,
and I guess they'll work for some readers and not for others. I expect
Japanese writing has just as many odd or archaic turns of phrase which
we completely miss in translation. :)
This should not be taken as approval of gratuitous Japanese phrases in
English-language fan fiction. That is a technique that I thought was
cute ... once. Not any more, though.
Cheers,
Angus