Hello Italo, your description is exactly right, and now I see the operating manual already describes this exactly right (from the 5.3 operating manual, p. 154):
There are several "secret keys" that can be depressed during startup to perform special functions. These keys should be pressed and held down during the second "startup screen" – the one with the "H8000FW' in large text. You can release the key once the appropriate "special screen" appears.
So I was doing it wrong! And thank you for your bonus insight about the tests the H8000FW is doing during the first startup screen (before the second startup screen that says "H8000FW"). The H8000FW is necessarily complicated to be as flexible as it is, but it's always very logical (especially since Orville OS 3 collected the clock page of parameters in the SETUP area).
Long story short — the startup options work the way you say!