I'd agree. Given the symptoms, the likely suspects are either an open circuit fuse on the +5V_USER supply, or a damaged shift register, U975.
The action of the left and right mouse buttons is independant of the movement logic; they simply pull the left/right input to 0V. So this will of course work without the 5V supply, which is required mainly to power the mouse quadrature encoder.
Checking the fuse is hardly rocket science, neither is replacing it if you can solder correctly.
I've written some brief notes on related subjects
here.
