I wonder if it's worth learning VoiceOver as a help tool to repair Macs and iOS devices with no-video issues. Any experiences regarding this?
Probably, but VoiceOver is a complicated beast. I really wouldn't wish learning it on anyone if they didn't have to. It's probably just easier to hook up an external display over HDMI/thunderbolt/USBc or mirror the display to and appleTv. But if you feel it necessary, go for it. Apple has a good help guide on it. It is a bit out of date, but the basics really haven't changed. Applevis.com also has a robust introduction to MacOS and VoiceOver.
I'll answer myself this question after I've done a bit of research for the Macs (not for iOS devices yet):
There're a few Voiceover commands that can help speed up the troubleshooting when the mac has no video.
However, none of these is as useful as connecting an external display.
To start with, you need to learn these easy basic voiceover commands (keyboard shortcuts):
Command + F5: turn on or off Voiceover.
Command + ALT: this is the default shortcut to use voiceover commands, just hold these keys (keep them pressed) while also pressing any of the other keys explained here. As these can be changed in Voiceover preferences, I'll just call them "VO".
Here's a few cases:
Case 1: you want to know if the keyboard is working without having to hook an external display, or instead of invoking the keyboard viewer:
VO + K: now type any key and you'll hear it pronounced. VO + F: does the same (this is the command to Find).
Case 2: you want to know in what power state the Mac is, such as turned on and logged into macOS/ in the log in screen / in the lock screen / in the recovery screen/in the macOS install screen: Note: this won't help in other "silent" power states such as turned off/startup manager/verbose mode etc:
You just need to get familiar with what voiceover says on each of these:
Mac is logged into macOS: VO + M (this gets you to the Apple menu on the top left), then VO + down arrow (gets you to About this Mac)...you can even thet the serial number if you use VO + arrow keys to navigate to the serial number, etc.
Mac is in the log-in screen (i.e.: logged out): use VO + arrow keys to navigate around the login screen: the login screen is different to the lock screen in that it also shows the Sleep/shut down/restart buttons at botton, and shows the time and WI-FI bars on the top right corner. So you just need to navigate around with VO + arrow keys: VO says: “voiceover on application – login- window – users – scroll area..” “sleep, restart , shutdown, 20:23, WI-FI 3 bars you may need VO + Shift + down arrow to start interacting with any of the spots you're on.
Mac is in the Lock screen:
The lock screen doesn't include the “sleep, restart , shutdown" buttons.
VO says “voiceover on authentication – login panel – window – enter password – secure edit text has keyboard focus.
With the VO + up arrow says you'll hear “user name, such as John Appleseed”, VO+up arrow again: “user image”. and nothing more, the lock screen interface is quite simple.
Mac is in the Recovery screen (the one you get by starting with Command + R): VO says “voiceover on macOS utilities – window – application selection. Then you can use VO + M (i.e.: the Apple menu) to navigate to the utilities.
Mac is in the macOS installation screen, the one you get after install that shows "Choose Language...etc": VO says “voiceover on language user-window- language user table
Hope this is useful, and please correct me if I'm wrong on something or suggest any improvements.