You really put a lot of effort in this, thank you.
Well, I found out, the series use reverse order, they are painted from Last to First.
A programmer always paints the background first, and everything else on top.
But when adding series to the chart, I have to Add the "background" series Last.
That's against my intuition.
And also against yours, since you wrote:
"you should add the line series after the range and the line will be painted over the area"
That's wrong. Your screenshots are the proof.
I hadn't set the color of the RangeSeries,
As you confirmed just now, and what I explicitly asked for, is "the default colors are transparent".
(this is not so simple to find out, since the Default Colors are applied later, at no point in your code, you can set a breakpoint and just inspect Series.BackColor, it's always Color.Transparent)
So what I had, was my black line, behind a (coincidently) grey area.
With all Colors being transparent by default,
the order of Adding things to the SeriesCollection, has actually only little influence to the result. You always see everything.
So the counter-intuitive Adding Order is just pointing out, when handling colors manually.
We finally clearified this, maybe you find a good place to document this.