Mid-staff key signature added before repeat barline (rather than after)

IIRC that was a deliberate change, to match what Elaine Gould wants to to look like
Probably together with #77396: Allow clef change after barline (for repeats)

See also #35076: Cautionary key/time signature before repeat barline if start repeat is in different key/time

Indeed, MuseScore 2 had this wrong, that bug is now fixed.

Although I agree it's logically that's what happens (the change occurs before the start repeat bar), my teacher/music producer says that in the book of music theory she studied the changes are inside the repeat block and not before the start repeat bar.

And there is at least one more user on this thread (mike320, see https://musescore.org/en/node/294313#comment-944731) that is unsatisfied with the default behavior besides my teacher.

I would like to be possible to modify the score's style (not a property on the clef, key signature or time signature itself) so the notation for clefs, time signatures and key signatures could be changed from the default (before the start repeat bar) to inside the repeat block, even if there is no change inside the repeat block after the first change. This way the entire score is consistent.

I've just encountered this. It's no good - I've a time signature change within the repeated section. On getting to the end of the section and returning to the start of it the player really needs a time signature to the right of the start-repeat ba-rline.
Possible semi-work-around: if the repeat section starts on a new line, then the signature is rather more obvious to the player, even if not in the logical place.

Indeed, the default is wrong in that special situation, so in those cases, you will need a workaround. Note you'd need it either way if the next section if not in the new time signature - the courtesy time signature before the repeat would need to be in parentheses, and possibly the original time signature confirmed afterwards, etc. So in order to solve this more generally, we'd need some new facilities here. Meanwhile, though, the workaround I'd recommend pursuing would be adding leading space in front of the barline if necessary (using the Inspector), then adding the time signature as symbols ("Z" to display that palette).

This forcing of the key signature to the left of a start-repeat barline may be "by design", but this is a rare occasion where I disagree with the rules set out in Elaine Gould's "Behind Bars". Surely it should be a standard notation option to show a new key signature within the first measure of the repeat?

As evidence, here are repeats in two different editions of Brahms Op.22 No.5 "Der Jäger" - and both editions show the new key signature inside the first measure of the repeat:
First edition: J. Rieter-Biedermann, n.d.[1862]:

Edition: Breitkopf & Härtel, 1926-27: