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The context toolbar should be just below the main toolbar. If you do not see it, open the View menu & make sure there is a checkmark next to "Show Context Toolbar."
Note that the content of the context toolbar changes depending on the tool selected.

The same thing applies if you use a rectangle to create the black outline: set its stroke alignment to inside (middle icon in the "Align:" area of the context stroke dropdown) so you can see it. If the photo itself does not have a white border, you can fill the rectangle with white & move it to the bottom of the layer stack. Then place & (re)size your photo however you want it.
Note that this is totally non-destructive -- you can change the thickness or color of the black outline or the border color to whatever you want at any time.

Also not true for me on my iMac. I am using the retail 1.9.3 Mac version of APub. Using your dancers pdf as a test, I get exactly the same info in the Resource Manager as in your screenshots.

The screenshot posted by the OP is obviously from Olivio Sarikas' "7 best Beginner Tips for Affinity Publisher" posted at 
https://www.youtube.com/watch?v=zFqKijjI4qc
When I launched the YouTube video it defaulted to 480p, but the 1080p HD version can be selected from the cog icon. On my 24" monitor it is adequate for learning when played fullscreen.
Olivio's video shows the full APub UI. If you run it full screen in HD, then that is how APub itself will appear on your screen. It is not the fault of a poor tutorial recording.
The size of the UI text elements is what it is in Affinity products. There have been many discussions in these forums about the minuscule typeface used by Affinity products. For many people, the typeface is simply too small to be usable. There are some Windows adjustments that will enlarge some of the UI  text elements, but not all.
 

You are right on both points. I think when I first ran this project a few years ago (??) the forest image downloaded at 300 DPI, so it was around 20 x 13 inches, but when I just tried downloading it again it is 83 x 55 inches at 72 DPI. Converting the DPI to 300 before copying it makes it work much more like the workbook says it  should, noting that it is not intended to fit completely into the spread anyway.
It may help to compare the document being created to the snapshot named Background in the search_the_woods.afphoto downloaded as part of the main project resources download.

I’ve just tried it again on iPad, starting with ellipses, and I also end up with ellipses. I think I must have started with circular ‘Curve’ objects to get my earlier result.

Assuming I followed your steps correctly, I also get a surprising result on my Mac, except that for me the child objects are ellipses:

Compound divided.afdesign (with history)

It is not scalable but if you double-click on either of the two overlapping color wells the Color Chooser window will open. Its HSL Color Wheel mode includes a much larger color wheel.

I don't know how he made it but it appears to be just one open curve -- note that there is only one red node.

FWIW, if I open your 300dpi.jpg in AD & export it back to a renamed JPEG file, APub's Resource Manager correctly identifies its original DPI as 300. Placing your file directly in APub shows its original DPI as 72.
The only thing I can see that is different between them using Preview.app's Inspector is the AD export includes JFIF density data:

Your original does not show a JFIF info category, so my guess is that is what APub is looking for.

I am not sure what "job" you mean, but if it is to provide some sort of legal protection of your design, I can only repeat that you should contact the library or a legal professional about that.

I think you need to contact the British Library & find out what they suggest you do, particularly about any design that in physical form cannot be accurately reproduced in electronic form.

I definitely do not. An actual reflective surface reflects light from the environment, so at best a simulation would show what it looked like in just one environment & only when viewed from one angle.

I have participated in topics about splitting the forums since around the time the first Windows app was released. To the best of my memory, there is not now nor has there ever been a strong demand for this, nor any consensus on how many sub-forums there should be among those who have expressed an opinion. Some say there should be separate sub-forums for Mac & Windows users; some that it should be based only on application. Some have suggested that there should be at least one 'general' sub-forum for questions that are not specific to one app or to one OS. Some have suggested mandatory tagging instead of sub-forums; more recently topic prefixes have gained a number of advocates.
Those in favor of splitting the forums contend that the apps are sufficiently different to justify this; those against it contend the commonality among the apps  is far too great for that. From time to time debates about if any of this would make it easier or harder to find the best existing answers, or increase or decrease the total number of topics become part of the discussion, yet there is never any consensus about that either.
To say that the staff don't care what users want is neither accurate nor fair. It is simply that we want different things & cannot agree on what would be best for everyone.

For me the Transparency tool works fine on "(Image)" layers & even on embedded document layers. But it does not work on "(Pixel)" layers.

In the Affinity implementation I am not sure there even is a default brush size as such -- as soon as the size is changed by any means there does not seem to be any record of the previous size stored anywhere user accessible. In effect, it becomes a new brush every time any of its properties are changed.

You are attaching far too much significance to that. Just because a post begins with a quote from someone does not mean everything (or anything) in it is about the person quoted, or for that matter about any person in particular.

In Affinity, the equivalent of switching brushes is changing from one brush preset to another. It isn't ideal but it does ensure that the user starts with the same brush each time.
Of course, digital artists using drawing tablets can choose among a multitude of raster brush presets that change their widths, hardnesses, opacities, etc. on-the-fly in response to pressure changes, & even when using a mouse some presets respond to velocity changes, so if it is essential that no brush properties change, it makes the most sense to me to pick among brush presets that have no dynamics & avoid using the keyboard to change any of their properties.

FWIW, the only builtin business card preset I have in AD v1.9.3 on my Mac is 88 x 55 mm, so it does not seem to match any of the standard sizes very well.

Is that a typo of varnish or do you really mean "vanish"?

FWIW, at least on Macs you can select more than one Layers panel item & "Add from Selection" will add them all to the subcategory.
But it seems that if a selected item is a "(Layer)" layer (the container kind of layer) each item in it will be added as a separate asset. Groups are added as single assets, with their layer structure intact.
This difference can be significant if by accident you select a "(Layer)" layer with lots of individual layers in it & add that, because annoyingly while you can add more than one asset at a time, you cannot delete more than one at a time. 

One additional annotation: you can also add pattern layers to the Assets panel. That are layers that will always appear as end- and seamless patterns if you place them on your image and drag it as you want. That is pretty cool. But in this case it will only work with afphoto files or with Assets you added from an opened document, not with JPEGs, PNGs and so on. Just as an additional hint.

It seems to me that the program is just using the Black that the user chose from the color wheel.

On a Mac, there is nothing faster or simpler than using either the File > Close menu item or its keyboard shortcut.
You could also click on the close button in the 'traffic light' window control group that every Mac app includes in its document window, but if you are not in Separated window mode, this will hide everything attached to that window, including the main toolbar, docked Studio panels, etc. That can be confusing so I do not recommend making a habit of doing that.

Then invest some time and learn to understand how Affinity vector clipping works. Once you've got a grip on it, you'll be wondering why you'd even wasted so much time and effort on pixel masks…