OK, here's what I would do.
Use Amiga Test Kit for the keyboard test feature.
Then use the following resources to understand the keyboard matrix:
https://github.com/BleuLlama/AmigaSc...yb_Mitsumi.pdf
and translate that to the A600 schematics:
https://www.amigawiki.org/dnl/schematics/A600_R1.5.pdf
And use amigapcb.org to figure out where those signals are on the A600 motherboard:
https://www.amigapcb.org/
(although, this only has it for rev 2 A600, the pinouts for the keyboarfd connector and MCU should be the same).
For example, the Left Amiga key (LAMI) is in the PB6 trace in the keyboard matrix. This is active (i.e. pressed) when it is connected to ground. PB6 is connected to pin 14 on the keyboard connector (CN13) and pin 26 on the keyboard MCU (U13). Use amigapcb.org to locate where these pins are and you can short it to ground to simulate that key press. If the keyboard MCU is working then the Left Amiga key in the Amiga Test Kit keyboard test should be highlighted. Repeat this for other keys if you want, by most of the keys require for the 'X' and 'Y' signals to be shorted, as per the keyboard matrix.