thats a hard drive, the problem will be on the logic board
USB mouse gets steady light?
CPU VCORE drops from 1.15V to aprox 0.8V?
Any reaction at AXG voltage?
No drop at VCORE, board is not functional; no need to check USB mouse.
Check around SPI chip, for corroded resistors, traces.
Don't forget R7403...
Just for testing, swap the BIOS/SPI ROM chip from another 3115 onto this board. If it chimes, you have either bad BIOS or dead/faulty chip.
If you DO get a chime/boot, then load up a new BIOS with the original/correct serial number, MLB and a clean ME region. PRAM on 1st start.
If you DON'T get a chime/boot, then you can eliminate BIOS/SPI ROM chip as the cause, and replace the original chip back on the board.