Rainer I'm surprised you replied this way, for certainly you know that there is a proper way to notate such a hemiola. You can't just ignore the 6/8 meter and write 3/4 music en mass (i.e. not a sesqui-rhythm). You have to write 6/8 meter that sounds like 3/4.
No, you don't have to. Sometimes it would even be wrong to do so, because it would no longer mean the same thing, it wouldn't in general even sound the same, as I explain below.
Conductors rarely direct rhythms, they usually direct meters.
I agree, but there are of course exceptions, as the words "rarely" and "usually" admit. Usually 6/8 would be beaten in 2. But you would not beat a pattern which nobody is playing. Another exception is where there is no unique meter present. Different sections of the orchestra may have different time signatures at the same time. In such cases a conductor will probably beat whatever will most help those who need help most.
To spell it out, this should be written with a 1/4 note, followed by two tied 8th notes, followed by a 1/4 note. This way the 6/8 meter is reflected despite the 3/4 sound.
But that doesn't have a 3/4 sound, because the dynamic emphasis is not the same. In 3/4, the first beat of the bar is the heaviest, with the other two being lighter than the first but roughly equal to each other: DUM-Dum-Dum (or if there are 8ths present it would sound DUM-da-Dum-da-Dum-da). Contrast this to 6/8, where the first (dotted quarter note) beat is again heavier than the other, and if 8ths are included, we get: DUM-da-da-Dum-da-da. Now tie these pairwise, and you get DUM-Dum-dum, that is to say the second rhythmic element receives significantly more weight (because it "inherits" the accent which now cannot happen in its proper place) than the third (because there is no accent for it to inherit). Specifically the weight difference between the second and third elements is greater with tied 6/8 than with 3/4.
(Regarding the flutes you mention, I may not have seen other instances or what you're talking about).
I only mentioned the flute because you said the strings and conductor would be confused by bars 113 to 116, but did not add that the flute might be confused by what begins in bar 117.
Some examples from Brahms (the master of hemiola and sesqui-rhythms) would be helpful.
You make Brahms out to be a paragon of clarity. He's not above being a bit of a rogue himself in that department. In a situation where anyone else would write a quarter note tied to an eighth, he happily writes a dotted quarter, and this even if the tie in question goes across a bar line. What he gives us is the note (or it might be a chord) just before the bar line, and then the first thing in the next bar is the dot(s) belonging to the aforementioned note (or chord). Apart from being confusing as hell to someone seeing this notational quirk for the first time, until one eventually works out what the devil it means, it also looks really odd because the dots are of necessity at quite some distance from the notes to which they belong, and don't trigger one's trained visual pattern matching mechanism, which is accustomed to dotted notes having their dots right next to them. I suspect he first did it as a joke, then liked it so much that he did it a few more times elsewhere. If you haven't come across this, I'm sorry but I can't now remember where specifically I've seen this.