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@,,, and @Hangman, you've both been incredibly generous with your help. Thanks to both of you, I now have a better understanding of colors, masks, and gradients within Affinity Designer.  Thanks again!

OK, I did say the document is CMYK.
However, I did not say the two shapes are "using the same base CMYK green". I said the back shape is a lighter green than the one darker green that is specified in all nodes of the gradient fill of the front shape.
 
 

There is a selector for the mode immediately above the sliders in the node colour editor.
It's at arrow 4 in the click sequence shown below.

I selected by dragging a marquee around all 3 shapes on the canvas with the Move Tool, which selected the group, so I did not think about selecting anything in any other way. Sorry I was not clearer about that initially.

Sorry for my confusion about who wrote what.

Now I see the anomalous value. If you had initially said Group instead of explicitly saying the three objects, that would have helped.

I said no such thing. I wrote nothing about anything except the "bots" typo until you got confused about who wrote what and asked me about cmd-R and rulers.

Suppose I have a path in designer and have used the pressure curve to apply a tapered stroke profile to it, like shown in the attached image. For example, I set the width to 50pt and then taper from 100% to 25%. If I cut the path in the middle, each half has the same overall style as the original, i.e. both have 50pt weight and taper from 100% to 50%. This is not what I’d like. Instead, the split paths should maintain the original visual appearance as the original path. The first path should taper from 100% to 62.5% and the second path from 62.5% to 25%. This would be an extremely desirable feature to have. It should look at where the original curve was split, then split the pressure curve similarly at the same ratio along its length.
Now, I am aware that I could expand the stoke of the original path first, then split it, and this would maintain the visual appearance. However I do not want to do this, for a few reasons. I want to be able to edit the path subsequently, and I need to be able to expand the path after the split (this relates to creating interlacing knot works from self-intersecting curves, by expanding stroke and then using the shape builder).
 
 

Summary: If I have a single self-intersecting curve in Designer (“D” in the attached) and expand the stroke, it produces “E”. The intersections have been erased/joined, however its best to describe. What I’d like it to produce instead is “I”, where the edges of the stroke at the intersections are left alone.
This all arose from my experiments using expand stroke then shape builder to build up interlacing knot work designs. If all the curves are discrete, “A”, then this works great, and produces “C”. However, if any of the curves self-intersect, like “D”, the it fails, because expand stroke doesn’t produce the stroke edges at the intersections. I hope this makes sense.
A fix is to deliberately split the stroke somewhere and then expand the resulting curves. However, there are certain situations where I can’t do this (namely, where I have used a tapered stroke profile) and would really prefer to just expand the one curve directly.
If it was possible to add an option to control how expand stroke behaves at self-intersections, it would be awesome. It could either do the default behaviour, where it simply outlines everything that appears black, or it actually traces the edges of the stroke.
ADDITIONAL: A similar option would also be invaluable for the contour tool! 

The three shapes have a Contour applied by Contour Tool. For some reason, there is no option to automatically scale the width of Contour when an object is scaled, even when the scaling maintains aspect ratio.
To get the gaps between the objects to be scaled along with the objects, you will need to first bake the Contours, either by the Bake Appearance button in Contour Tool or by applying Convert To Curves command.

Exactly! That's what should happen when a Group has Passthrough blend mode (or 'Pass Through' in Photoshop). The name describes the expected behaviour.
Unfortunately, Affinity started with a dual-personality Passthrough and the developers have stuck with it despite many complaints in the forums over the years.
There is a hack that persuades Passthrough to consistently behave as expected: make the bottom object in a Passthrough group be white with Multiply blend mode, and cover the entire canvas with it. A white Fill Layer or vector Rectangle are ideal.
 
 

IMO, Serif should add an icon to the layer when Blend Options adjustments are not set to the default values – like they do with Layer Effects.  Currently there's no indication to the user at all.
 

Maybe I misunderstand you, but toolbar alignments do make use of the key object, and the alignment popup of the main toolbar considers the key object to be the 'first selected' regardless of when it was selected.

For me using the app's dark UI, the button background is darker when enabled.
To be clear, my opinion is that the status of that option should not prevent opt+click from being effective in the document view, and I've now reported it as a bug:
 

Found it. Selecting the key object by opt+clicking on canvas is not possible when 'Allow selection to consider items inside a group' is enabled. Smells like a bug to me.

For me using the app's dark UI, the button background is darker when enabled.
To be clear, my opinion is that the status of that option should not prevent opt+click from being effective in the document view, and I've now reported it as a bug:
 

Hi @,,,,
This is a known issue that's already with dev. Thanks for raising it up though.

Found it. Selecting the key object by opt+clicking on canvas is not possible when 'Allow selection to consider items inside a group' is enabled. Smells like a bug to me.

You need to option+click the object's thumbnail or name in Layers panel.

Hi @,,,,
This has been previously logged with the developers, i've bumped the issue with the details of your report.

In version 2 apps you can select all objects to be used in alignment, then use Alt/Option key to select the object to align to (it will be highlighted), and finally use the main or context menu alignment command to align the objects:

alignto_object.mp4 As far as I know this only works with menu commands. When using the Alignment panel, use methods shown above.

I know that Affinity does its own thing, but I see this as a very handy feature from photoshop that makes the workflow so much easier when working with brushes. It feels very alien to keep going to the top bar to make settings and it slows down the development of my work. I don't see it as something impossible to implement, and even though this is a Photoshop feature, it's always great to take the best practices and apply them in your software as well.
Hope to see this feature in future updates. Thanks!

The Alignment popup in the main toolbar has target options including "first selected" and "last selected".
The Alignment Handles device, which can be enabled in context toolbar of Move Tool and several other tools, has a right edge handle for aligning the right edges of all selected objects to anything, and it observes many of the available snapping options.

Try ensuring Transparent Background is not enabled in the Document menu and then export to 16 bits per channel RGB TIFF.

Try ensuring Transparent Background is not enabled in the Document menu and then export to 16 bits per channel RGB TIFF.