I read through the two dell.com threads you linked, and unfortunately the amount of misinformation there made me cringe. There were multiple users who wrote long paragraphs about the 5V and 3.3V rails with 100% confidence, not realizing they have zero understanding of how they work. I'm afraid there's nothing salvageable in those threads and they didn't help you with your concerns at all, perhaps even confused you more.
Yes, you're correct - it's that simple. The 5V rail powers only a small range of devices, like the ones you described. If your parts that use 5V load this rail above its stated limit, your PSU will shut down at that moment. If you install a lot of hard drives and try to spin them up all at once, you may come close to this limit. However as you've seen with your own calculations, reasonable setups don't need anywhere as much 5V power to come close to the limit on your power supply.
The 3.3V rail is used even less, and the amount of power components pull from it is absolutely minuscule. It's not worth worrying about at all.
No, the maximum output of the 5V rail isn't in any way related, and doesn't affect in any way, the operation under cross-load.
In group regulated units, when the 12V rail is heavily loaded and the 5V rail is barely loaded, the 12V voltage raises a lot (possibly over the maximum 12.6V allowed by the spec) and the 5V voltage drops a lot (possibly below the minimum 4.75V allowed by the spec), regardless of the maximum rating of 5V. The maximum rating on 5V is not in the equation. The maximum rating on 5V only dictates how many components that draw 5V can you install in your system, and is chosen by PSU manufacturers according to what they believe users will install in their PCs.
Yes, you only worry about the 5V rail if you expect to draw a lot of power from it. Processors and graphics cards use 12V, and barely any components use 5V, like you already know.
You are correct, that's bullshit. Specifically, graphics cards can draw up to 5.5A@12V from the PCIe slot (that's 66W) and up to 3A@3.3V from the PCIe slot (that's 9.9W).
I leave it up to you to decide if 9.9W is too close to your 3.3V rail's 66W limit (3.3V * 20A), or too close to the 100W limit of 3.3V&5V...
Completely wrong.