Your ideas are great and educational, but it's an overkill. An engineering approach will be to achieve the tasks with minimum resources (cost and effort).
You want me to intercept the bidirectional data flow between the PS2 mouse and the PC and modify it to detect or add a key press while hacking into the mouse buttons achieved the same results without any code!
My mouse buttons send 5VDC to the mouse control chip, while they have pull down resistors.
What I did successfully was send 3 wires from the mouse, 2 for the switchs outputs and one for gnd. connected mouse gnd to microcontroller gnd and used 220 Ohm resistor in series to protect microcontroller pin in case it was low and the switch was pressed supplying the port with +5VDC.
Now I can monitor keypress or control the key easily even without relay or transistor.
Mahmood