How are you measuring the mouse input at U15? The mouse outputs quadrature wavefoms, which is essentially a pair of square waves for each direction, where the frequency of pulses relates to the speed of mouse movement, and which half leads/lags the other relates to the mouse direction.
In other words, if moving the mouse diagonally, there should be square waves on all of the four movement lines. The duty cycle of the waveform is approximately 50%, so using a DC multimeter, you'll see an average of about 2.5V while it's moving. Each of the four inputs will randomly stay 'stuck' at 5V or 0V when the movement stops. You ideally need an oscilloscope to see what the signals are doing.
The four movement inputs for the mouse are the same as for the joystick. So if the four inputs work correctly with the joystick, then the computer is probably OK. Use Keir Fraser's excellent Amiga Test Kit to easily see how the computer is reading the joystick/mouse inputs:
https://github.com/keirf/Amiga-Stuff/releases
