NOTATIO

How would you write this bar?
It's in 3/4 time, and I believe it is clear as is for musicians, but it clearly doesn't fit in 3/4. If one assumes the piano to be the correct one, the other two parts need to be heavily altered.
I will share the option I've chosen—for now—once I hear your opinion, to avoid influencing it.

Seems pretty clear 3/4 to me. Top instrument correct as is. Vocal hits the F on beat 3 then has a cadenza during the fermata up until the C, at which point everyone is back in time. Error seems to be in the Piano as the fermata really should be on beat 3, not beat 2, which is misplaced. It wouldn't make any difference in performance, and perhaps it's simpler to notate it that way, but I would be inclined to actually show it on beat 3. You could use some hidden tuplets I suppose if you really wanted to preserve the original.

It can easily be done in Dorico with one hidden 4:1 tuplet for the cadenza, and then everything lines up correctly with no manual positioning except for the 16th rest. A couple of superfluous fermatas just need to be dragged off the page. That's sorta hacky, but it is what it is.

Nice!
I initially assumed the piano to be right and placed a 2:1 on the first beat, then a 3:2 on the second, which adds more rests to the cello part.
Your solution is definitely more elegant!

To try to make the 16th of the cello align with the 8th of the voice and the following 16th contain everything else before the fermata, I tried to do a 2:1 16ths for the F and a 6:1 16ths for the rest, but Dorico would not let me do it. Upon writing 6:1 16ths it would overflow in the next bar for reasons I've not been able to diagnose so far.

Ah, that's a neat way to do it! Then you don't even have to manually align the 16th rest. I'm not sure why it's not working for you, but I managed to get it to work.
Although technically shouldn't the 16th rest arrive slightly before the start of the cadenza notes?

I will try again in the morning and post some pictures as well!
Why the composer wrote the F with fermata as an 8th instead of a 16th is a mystery

Now the 6:1 works, but until Dorico was quit and relaunched it would not cope.
Here's another solution, assuming the piano would be right.
Your second solution, which merges yours and mine together, appears to be the neatest one.

NeeraWM wrote: ↑08 Mar 2024, 23:47 Why the composer wrote the F with fermata as an 8th instead of a 16th is a mystery

A sixteenth might imply too short a hold. Judging from the rest of the notation, the composer wanted the singer to hold the F beyond the length of the A in the cello and only then start the cadenza. But there was a danger that the fermata over the F might be misinterpreted as a misplaced one of those large fermatas that Mozart and others placed over such cadenzas that apply to all the notes, not just to the F.

Incidentally, I am really glad that Dorico "syntax' allows x:1 tuplets. It has solved all kinds of issues of this kind.