[WRT:] ...connecting the Pi and the motherboard in this way is a bad idea, as it will be physically connecting otherwise isolated circuits with separate ground signals.
Am I right? Or is this a non-issue? Is it a bad idea for a different reason? Why?
The correct answer to your question as stated is:
Yes, you are right - THIS IS A BAD IDEA....
"I don't care about sacrificing the adaptor, but what's the chance I'm taking to mess up my Raspberry Pi by using a power adapter with the correct voltage but not enough current?"
Unless the Raspberry Pi is extremely poorly designed (which I very much doubt is the case) or unless the power adapter does not limit it's output to 5V, you will not ...
Actual hardware damage is very unlikely. The real risk is data loss: you can't trust your system to store data reliably, be it on the SD card, or an external storage such as USB thumb drives.
It's probably not a big problem if you just want to learn a bit of Linux. The moment you try to use the Pi in an actual project, it becomes a problem. You'll likely ...
I had exactly the same issues as you describe here. I was also following instructions from the book Learn Robotics with Raspberry PI. For my project, I was using pi 3 model b+. I solved the problem by replacing the battery holder with another one of better quality.
If it helps, this is the battery holder I used https://www.ebay.co.uk/itm/133616385476
The charger you have asked about should be fine. The USB-C specifications allow the higher voltage levels, and a "signaling mechanism" that is part of the USB-C standard guarantees that as long as the device (your RPi) and the power supply both adhere to the standards, things will work as they should.
That said: The first RPi4 production units were ...
So what would you tell someone who has no technical knowledge of electronics? Or how would you educate a casual hobbyist to give a confident answer when someone else comes to them as the expert?
I would tell them, "Don't be penny wise, and pound foolish." Buy a power supply that meets the specs. Using a power supply that cannot supply enough ...
If its pot-free/dry contact it shouldn't damage the pi, but if there is any kind of external power I would use an external relay as a switch. Use the pi to control external relays and you will be safe.