ok, UART debug up and partially running on my SIII mini: debug messages displayed on terminal but keystrokes do not reach the phone, this is secondary for me at the moment, I may have a bad contact somewhere.
Tested on my dead I9070: no display, so the Xloader on my eMMC is garbaged (or Xloader UART debug is disabled, but this is less likely).
As expected, I now have to figure out how to have flashloader boot files upload *and* debug working together to understand what's wrong with my compiled boot files. I think the "trigger UART" thing is a good track, but I'm really puzzled by how to have the USB *and* the UART setup at the same time.
I fear to fry something by having phone D+/D- connected to USB port of the PC and connected at the same time to my Prolific TxD/RxD + 5V VCC connected to PC USB... sounds like a bad thing.
Another track would be USB debug I see in some parts of the code, but I don't know how to read the debug from there, more code to inspect...