While C++ is not one of my strengths, All of my QML development has been done using Python, I would happily take this one on if I could understand how or where in the code the checkboxes in KCMs ui are parsed, and the values stored. Additionally if I could understand how or where the parms are passed from the C++ code to the QML.

Would yo be able to comment on why you want this option? What use case does it support? What's the problem with the status quo? Those are the kinds of questions we need answered before we consider adding a new option or feature.

1/ In my case, least I prefer to have a set number of virtual desktops, and ...
2/ The functionality is already available on the Desktop Grid.
3/ In order to get that functionality on the Plasma 5 Overview I have been editing the QML and removing the buttons. I have a small script that takes care of system updates.
4/ Adding that functionality to the Overview would make it consistent with the Desktop Grid, and achieve the same or similar affect.
5/ I think it would be useful for those, who like me prefer the set the number of Virtual Desktops in the Workspace Behaviour/Virtual Desktops area.
6/ My personal reasons for having a preset, and 'immutable' number of desktops, is so I can open different Applications on predefined Virtual desktops, and I, personally, at least, don't want to accidentally remove a Virtual Desktop, when navigating around either Overview or Desktop Grid. Nor do I want to add Virtual desktops, once I have things configured.
7/ I believe others will find this useful.

8/ Given that there is already the functionality built in to Window Management/Window Rules. Where it is possible to nominate an existing Virtual Desktop to open a Window on, not being able to ensure that Virtual Desktops are more or less 'immutable' breaks this functionality. 
9/ Currently being able to nominate that the Add/Remove buttons are disabled on the Desktop Grid compliments the Window Rule that a Window open on a specific Desktop, by ensuring that, that Virtual Desktop is always available. To make this available on the Overview would ensure the same thing when the Overview is being used, currently, of course, this is not the case with Overview.

The fact that you can add and remove virtual desktops on the Overview but not the Grid View is indeed a bit weird, yeah.

I'm a bit ambivalent about making the ability to do this optional, but maybe it couldn't hurt if the toggle controlled the UI for this in both Overview and also Grid View.

Given it appears they have been somehow merged, in Plasma 6, although as far as I can tell they appear to have separate code sets, I think that would be good. In Plasma5, which I'm currently using, they are separate and the Grid View has the option to disable Add/Remove Virtual Desktops, but the Overview does not.

I just checked, and yes, in Plasma 6 there is no option to disable Add/Remove Virtual Desktops, but, as you say the Add/Remove Virtual Desktops buttons are not available on the Grid View any way.

As I have already indicated, I would like to help.

Thinking about it more, I wonder if we might even want to go in the other direction and remove the ability to add and remove virtual desktops from both Overview and Desktop Grid. People who use virtual desktops generally have a fixed layout they've already carefully set up in System Settings, or else they use a 3rd-party KWin script to automatically add new virtual desktops as needed. I wonder what the actual use case is for being able to manipulate the number of desktops here--especially since you can't change the layout, which makes the feature semi-useless unless you stick to a one-dimensional layout.

CCing Niccolò for comment.

(In reply to Nate Graham from comment #7)
> Thinking about it more, I wonder if we might even want to go in the other
> direction and remove the ability to add and remove virtual desktops from
> both Overview and Desktop Grid. People who use virtual desktops generally
> have a fixed layout they've already carefully set up in System Settings, or
> else they use a 3rd-party KWin script to automatically add new virtual
> desktops as needed. I wonder what the actual use case is for being able to
> manipulate the number of desktops here--especially since you can't change
> the layout, which makes the feature semi-useless unless you stick to a
> one-dimensional layout.
> 
> CCing Niccolò for comment.

Personally, I'd prefer to keep it. In my mind, the Overview is almost the more obvious place where to handle the desktops, even more than System Settings; especially the grid view is a good place where to learn about them, since it tells you what they are and immediately gives you the option of creating more. Other major OSs also handle VD entirely within the Overview, so I think users will expect to be able to do that. It's also much simpler to access (and, honestly, even more intuitive to use) than the settings page.
You do raise a good point about the layout. Personally, I'd move away from the idea of exposing "you have N desktops in M rows", and instead expose "you have N rows and M columns", and then we'd expose (in the Grid View) the ability to change the number of nows and columns. I'd still keep the ability to change the number of desktops in the Overview, for those with a more simple, linear setup. (If you have a grid you don't have any preview in the Overview anyway).
So, we have completely opposite approaches here :P

At present the Overview/Grid View works like this...

<img src="https://iili.io/Jv0ftMF.png" alt="2column2RowsGridconfig" border="0">
<img src="https://iili.io/Jv0fpcJ.png" alt="2column2RowsGrid" border="0">

<img src="https://iili.io/Jv0fD6g.png" alt="SingleColumnOverviewconfig" border="0">
<img src="https://iili.io/Jv0fLZP.png" alt="SingleColumnOverview" border="0">

<img src="https://iili.io/Jv0qJup.png" alt="SingleRowforOvervieconfigw" border="0">
<img src="https://iili.io/Jv0q2tI.png" alt="SingleRowforOverview" border="0">

My proposal is to simply make the Add/Remove Virtual Desktop optional, as it currently is for the Desktop Grid on Plasma 5.

<img src="https://iili.io/Jv0oe1t.png" alt="Current Optional Addremove" border="0">

Of course if removing the Add/Remove buttons is your preference, I would not argue with that, as it is easy to add and remove Virtual Desktops, by various other means, including the Virtual Desktops config panel.