When working with metric maps, make sure to set the symbol scale to 1 before placing them. Scale bars are scaled in map units, so they should always be scaled to 1 when placed to appear correct.

Other symbols are scaled for imperial by default, and should be placed at scale o.3048. This should already be set as the default value in metric maps.

To manually type in the size, just right click in your map with the symbol at your cursor. Here you can type in the desired scale for the symbol, or just hit the set normal button to go back to the default scale for that template. To scale things already in your map, you can also just type the desired scale on the command line when CC3+ requests it, instead of scaling by moving your mouse, this allow you to easily get the exact value.

The easiest is to follow the instruction @Raiko provided above.

But for some more context. The Change background color command is a legacy command from back when backgrounds were solid colors, before we started using bitmap fills. When an entity has a bitmap fill, the color won't be visible (because you see the fill instead), so this command will appear to do nothing. As such, just ignore it unless you work with a map with a solid color background.

As for the error message about "FREEZE BACKGROUND", the cause for this is that the macro behind the command attempts to freeze the background layer, so this is a command, not the name of a layer it looks for. However, if something goes wrong with the macro (like it can't find a background entity [not all maps have one]), that can lead the macro to interpret that line as input to the layer selection instead, thereby complaining it can't find a layer with that name.

The article is found in annual 129 - Large exports.

Not sure about the line in the compass. almost look like a miter line, caused by sharp turns between two nodes, maybe some overflow happens with the calculation at that export resolution?

You can't zoom to a specific % because that concept doesn't really exist in CC3+. CC3+ is CAD software, and the content is always rendered to whatever size you select, there isn't a "full", "print" or "actual" size.

With an image editor, zooming to 100% makes sense, because that means that each pixel in your image is being showed as the same size as a pixel on your screen, but CC3+ maps aren't expressed in pixels.

For print size, that really depends on what scale and paper size you want to print at. As with showing on screen, print isn't a fixed size, CC3+ renders the output to match your print settings.

Considering your screen is likely to be much larger than a sheet of paper (unless you are working on a laptop screen), zooming to show the same content that will be on your paper wouldn't really give a perfect view of how it looks on paper anyway, since the paper is so much smaller.

That's a drawback whenever you deal with fantasy. In classic fantasy, one has to suspend disbelief a bit in the face of magic, which can do stuff not normally possible. In steampunk, one kind of have to accept the same of their steam technology. It's basically their "magic". I guess it is quite similar as to how utterly unbelievable our modern world would be to a renaissance person (It's just that our version turned out to be possible under the laws of physics)

It's not that lego got involved (AFAIK), it's just peoples tendency to build everything out of lego. Lego is a great outlet for creativity, you don't need the lego company to make a set for you to be able to make nice things. These days, you can even build lego online on their website, making designs without owning a single brick.