Ideas for using "OCTAVE" in combination with note shift:
https://www.yamahasynth.com/forum/zone-master-octave-transpose-and-midi-single-mode-issue

Using Element Osc/Tune coarse tuning (also mentioned in above thread) - but summarized here on its own:

https://yamahasynth.com/forum/part-trasposition-more-than-24

I would personally also consider swapping the marimba and flute locations.

I don't have a Montage, I just troll here to learn about the newest beast. This sounds exactly the same as (Motif) - you can "tune" a Part (Voice) in a Performance +/- 2 octaves but to go further you have to tune each element within the Part (Voice) - where at that Element level you can tune an additional +/- 4 octaves. Total shift you can thereby make is +/- 6 octaves total, which will cover any situation a normal human should be able to conjure up.

Don't switch LH vs RH in how you play - program it to be where/how you want it. Program it once, play it forever as you want. Dive into the Part and course tune each Element the same desired shift and you'll be good.

No directive to switch the hands -- that's why I personalized that one to myself. I've had reasons to place "soprano" parts near the bottom of the keyboard and "bass" parts near the top. Usually this involves where the buttons or controls are that I have to use at the same time as playing the part. Maybe the "bass" PART I need to use the pitch bend frequently -- so I'll free up the left hand for that. Maybe the soprano PART I want to change ARPs and those button controls are on the right side of the keyboard - so I'll place the soprano PART in the left hand to free up the right. So choice of LH/RH is entirely up to the OP without any suggestion/option, hopefully, forcing that hand.

Less of a technical matter, but more philosophical - stretching your skills as a player has benefits. It may not be bad to make a [STORE] (non-overwrite) copy of the Performance which has the parts swapped and practice simply as an exercise (and not as a performance goal for this Performance). Only reason I'm pushing back on the "don't do that" suggestion is because it rubbed me as going counter to taking advantages of opportunities for musical growth. BTW: I'm not saying to go through this exercise or not -- just like to see options presented more than directives unless there are true pitfalls to point out (like don't play Montage with your keyboard unplugged from a power source).

... back to the technical side:

If/when you do note-shift the instruments - make sure you look at the elements' note limit settings. They do not automatically adjust to the shifting. When you shift around the pitches - the keyboard stays the same. However, some elements are only supposed to play above (or below) a certain pitch. This programming will not match when you note shift - also shift your limit ranges to match. If the range is full keyboard, then you can disregard. Many of the flutes, for example, limit which notes receive which "attack" sound. There's a different sound for one range of the flute vs. another (when playing at high velocity). Same may go for the marimba (I haven't checked).

I think it's ridiculous that Yamaha won't modify performance editing to allow higher/lower transposition of parts. It's often requested - it seems technically it should be easy enough to implement - but the most common reply to this is that a performance is not meant to be played with instruments way out of their typically played ranges. I really don't get it.

I did give specific rationale why one would play instruments in areas of the keyboard outside of the typical ranges - and even opposite the real ranges (putting bass parts up top, soprano parts on the bottom). I haven't seen typical range pushback here (yet).

What has been presented is you can already cover the 4 octaves requested (and two more) with the controls made available. Considering that the 16 bottom keys of an 88-note keyboard can be mapped on the top 16 keys pretty much covers the spread.

Yes, there's a small gap there - it'd be best to cover the full spread and add another 16 -- but there's hardly anyone ever -- ever -- that's asking for more than 6 octaves in one direction (with +/- , it's 12 octaves of shifting total). Once it's learned that what they want to accomplished can be done by going one level deeper in their programming - the question goes away (because it's solved, not because there's frustration over no solution) until the next user comes along who requires a refresher on the programming facts.

Knowing that "allow higher/lower transposition of .. [edit, adding: notes in] ... parts" is already a feature that exists - I think the remaining complaint would be that it's more difficult to edit elements (coarse tuning) because you have to adjust 3 things (coarse tuning, note limit high and low) for every element (up to 8 elements = up to 24 things) vs. just 3 parameters at the PART level (note shift, note limit high and low) total. Yes, this is true -- it's more difficult. Not impossible. A reasonable request to supply some way to ease this (maybe PC Performance editing software which does advanced things that are more difficult to do, yet still possible, on Montage itself - maybe a call for 3rd party development of this feature - maybe helping guide direction of what's important for the future).

Typically there are not more than a couple elements to modify. This does not tend to be a big deal to manage. It takes many more times to map out where the keys should go with just a pen and paper to orchestrate the dance on the keyboard controls than it does to enter all this into the keyboard as a programming session. With a 76 key keyboard - there's only 4 keys "out of bounds" to map from one extreme to the other - 3 if you count "fine tune" as squeezing out an extra semitone. Even with an 88, I wouldn't use the limitations to dictate the octave shifting pen-and-paper work. This is not likely to "bite" you when it comes to programming the note shift/coarse tuning.

Thanks for your responses. The element-level tuning is a great idea.

As far as choosing what keyboard range I need to place an instrument in, it's usually based on numerous factors including how frequently I need to play that instrument in the section of the song and the state of the keyboard in any prior and successive program changes - which may be numerous during the course of a song.