could I slice the PCB trace that enables autoreset and then flash using it?
If you install the latest version of Optiboot provided by @westfw you should not need to do anything special (like cutting the RESET trace) for the Arduino ISP Sketch to work.
However you would have to re=enable the trace path for when you want to do future uploads of other sketches onto the board, or learn how to time manual resets at the proper time to allow uploads to work.
Look for WestfW's Optifix sketch
Does Optifix update the processor on which it is running or does it update other processors?
How do I read the Optiboot version?
I've also uploaded fusebytes.zip, which pretty-prints assorted "deep" information about your Arduino. Including the optiboot version number if present and readable.
#define USEFLASHLIB 1
Bootloader at 0x7E00 is not readable