1) "Octave Lock"
This same goal could be reached by a different, also non-existent, path. To enable offsetting the actual "note shift" parameter. If this was available, then you could assign a controller like an assignable switch to alter the octave of only one PART while keeping the other PART(s) the same (or offset any collection of PARTs). There would be a lot of flexibility in this - not only octaves - but also transpositions.

The ideascale entry for this is here: https://yamahasynth.ideascale.com/a/dtd/Add-Note-Shift-Offsets-Internal-and-External/212248-45978

2) "Buying software to create waveforms".
Today, for Montage at least, there's the free (not buy) option of using Samplerobot which can export to waveform format. Samplerobot is useful for certain number of things - but it wasn't necessarily made to create a collection of waveforms to import to Montage. You can do this, but at some point some of the ways that Samplerobot work can get cumbersome as a generic waveform producing tool.

Before Samplerobot there was more to complain about in this category. Samplerobot has "sort of" taken the edge off. The other piece of software - the JM Tools Waveform Editor - and the only supported software for the year+ preceding Samplerobot availability - was annoying to have as an at-cost version considering that many (most) of the on-keyboard tools for manipulating samples were stripped out. Forcing the software method. Which, during that year, "forced" purchase of software if you wanted to edit samples/loop points/define waveform parameters. I complained "the heck" out of that decision - to take away a feature (relative to previous keyboards in the lineage) and not provide the safety net of a free software solution as was done with the sequencer (Cubase AI).

I guess what I'm saying is that I agree on this front. But if you want to do serious sampling work with Montage - you have a limited set of choices for doing it without a purchase. And that doesn't rub me the right way either.

3) "What's up with PARTs 9-16?"
Forget about scenes. That's an extra layer of "stuff". The fundamental here is how PARTs 9-16 are different than PARTs 1-8.

... below "piano keys on Montage" assumes just the keys and no interaction with external gear.
Rule: You can only play, at most, 8 PARTs at one time using the piano keys on Montage.
Rule: The 8 PARTs you can play all at once using the Montage piano keys are PARTs 1-8.
Rule: PARTs 9-16 you can only ever play one PART at a time using the Montage piano keys
Rule: If you want to play more than two PARTs at the same time using the Montage piano keys, you must satisfy the following:
a) Each PART you want to play at the same time together must have the "Keyboard Control" (abbreviated "Kbd Ctrl" on the HOME screen - with a greenish piano icon when enabled) icon enabled (turned on - greenish color, not grey).

b) You must NOT have currently selected a PART with keyboard control as OFF. If you are editing PARTs 9-16, then the currently edited PART (whichever between 9 and 16 you are editing) is the -selected- PART. If you touch the "mute" button on the home screen of PART 10 - then this will also -select- PART 10 as a consequence. Selected PARTs have a white outline around the PART. That's one way you can tell the PART is selected. There are others. You can make sure you have no (zero) keyboard control=OFF PARTs selected by deselecting ALL PARTs. This is done by pressing the [PERFORMANCE] (HOME) button which will deselect all PARTs. If you do select a PART with keyboard control=OFF, then only that PART (and no other PART) will sound when you play local Montage piano keys.
Rule: PARTs 9-16 force keyboard control=OFF. You cannot turn keyboard control ON for these PARTs. That's why they're different.

To me when you say only PART 9 is sounding - this is probably because you have PART 9 selected and, by the rules, this means no other PART will sound when you play local Montage piano keys. There is no way to make PARTs 9 AND any other PART to respond together with local Montage piano keys.

Note: when I say "no way" I mean assuming you have no external device which is interacting with Montage. There may be a way to setup external gear to respond to what Montage is doing and the external gear can then target the Keyboard Control=OFF PARTs.

Scenes build upon this fundamental - I think the fundamentals are where the understanding needs to be at the moment.

Concerning the octave lock: I have the same issue sometimes. My solution which works now if you still have free parts is to have the part which you want to transpose in two versions which have different transpose settings. I use them in different scenes by enabling keyboard control for each only in one of the scenes. That works quite well, especially with OS 3.0... That's actually nicer than having the locked octave control because the scene knobs are better reachable for me.

Excellent, Stefan! That’s using the technology. The Octave buttons on the front panel Transposes the Keyboard... The “octave lock” (sic) request is a request seeking to change the Tone Generator but that is not what those buttons do only. Yes they change things but not specifically the item you want to change. While what’s at the heart of the request is musically understandable as a real world request, the difference between Keyboard Transposing and your solution, which is actually changing the Tone Generator Part receiving, is huge! And is the key to understanding how it works.

“Transposing“ the Keyboard, as Yamaha is referring to this, can be easily understood because in a piano/ bass split you only have one keyboard. Changing it up or down shifts everything (both instruments) up and down.

You want to shift just the right hand sound, you can understand that is NOT an adjustment of the Keyboard (it will do both), but an adjustment to the Part - the Tone Generator - for the right hand sound. Changing the Tone Generator Part is referred to within the product as NOTE SHIFT (as a way to differentiate these two similar, but very different functions).

The Keyboard Transmits, therefore changing the Transpose parameter is transmitted Out via MIDI
The Tone Generator Receives, therefore changing the Note Shift parameter within a Part is only local to that one Part, it does not effect what goes Out via MIDI. Note Shifting the Piano Part does not impact the Bass Part.

Suggestion:
To save your request from landing in the “doesn’t understand” bin... clarify your request that often you need to change the tuning of one part of multi program without affecting the other.

You can imagine the reason the Octave buttons shift the entire keyboard is because that is the most popular use for the feature. Out of 100 pianist, how many even play a split keyboard? The average player is playing a Song, they only know in the key of “C” so they transpose the entire keyboard this way they can still play it in the key they know.

When you Split the Keyboard you move into the *specialty* zone. If you’re clever enough to Split the Keyboard, you should be clever enough to realize each Part is individually tune-able, each Part is playing from the same set of keys, if I need to change just one of them I need to address them individually. You probably only want the Sustain pedal to sustain the piano (right hand) sound... again when looking to set that up you know to go to the individual Part to make the adjustment. Naturally, stepping on the Sustain Pedal holds all the notes the one Keyboard is playing, to prevent the left hand bass to not sustain you must edit a Part parameter. Similar thing.

So if you are going to take the time to make a request, show you understand how it works, and if you can extend the thinking a bit, try to understand why it works as it does... then finally, if you can, make a request that would serve your desire for workflow... on the HOME screen channel strip for a each Part might be a Note Shift icon addition, for example. Or with the Scene Memory an item that memorizes the Note Shift per Part, etc., etc.

WHY?

The MONTAGE can play a maximum of 8 Parts at a time (simultaneously).
Each Performance can hold 16 Parts... this is not difficult to understand.
If you place 16 single Part instruments in a Performance... you would press [PART CONTROL] now the top two rows of right hand front panel Select buttons give you direct access to Parts 1 through 16

If you wish to play several Multi Part Programs you would set them up in Parts 1-8. No, you cannot link Part 1 with Part 9, if that is what you decide you need to do, please use the EXCHANGE function to swap Part 9 with a slot numbered 2-8.
EXCHANGE = [SHIFT] + [EDIT]

Any Parts 1-8 can be configured in pairings to your requirement. Any combination of the first eight Parts can be recalled.
Please see the two articles on “Performance Basics” for ideas about how to configure your own custom Performances.

It’s less challenging than a game of chess or Rubik’s Cube, but you can figure a way to setup the sounds you need to play. Exhaust all possibilities before deciding it can’t do it or that the only solution is being able to layer all 16 Parts... probably good reasons that doesn’t happen.

Hope that helps.