... I don't think Jim has stipulated that the composer who was the father died young. However I am not up on WOJs.
The exact quote -- and the only one I know of, though he may have dropped new tibits since -- is from an AMA in 2014:
Of particular interest Maeve and Sarissa, were they actually Mabs kids (biological sense)? If so, who was Mabs baby daddy?
Mab and Titania are actual twin sisters.
Maeve and Sarissa were twin sisters, from Mab. Their father was an Austrian composer and musician who died young.
-- https://wordof.jim-butcher.com/index.php/word-of-jim-woj-compilation/woj-on-the-fae/Derived from here:
-- https://www.paranetonline.com/index.php/topic,50771.5/wap2.html
@Serack (or whomever) is no longer transcribing & collecting Jim's AMA's and signing-events &c (or at least, not that I know of). So, in some newer repository maybe <crossing the streams> the truth is out there. </streamcross>
... If he did, why didn't Jim name him?
God only knows! Well, God and Jim Butcher, presumably.
But we notice that Jim does seem to love dropping clues & fantheory-fodder... I presume this is another instance of that.
... Austria has had more than one composer of note, great ones, however it doesn't mean that Mab would choose one of them to be her lover ...
I would argue that Mab would
only choose someone of "great talent" (even if a lesser-known one to mortals). Maybe a genius produced only a few pieces -- or just one -- before dying; if it was of sufficient genius, it may have caught Mab's attention (even though our history books note him as just a minor figure).
Via Google, I see there was a reddit thread whose majority seems to have decided "Mozart too obvi, gotta be Schubert!" Which... c'mon, man, Schubert ain't chopped liver! If "obvious" is a reason to discount someone, better discount Schubert too!
HOWEVER: there is
this little piece by Schubert:
https://www.youtube.com/watch?v=JS91p-vmSf0(just sayin')
But your core point remains valid, Mira: could be some minor-footnote guy; could be an "Anonymous," name unknown to modern musicology. We don't
know, we have no proof one way or the other.