Gave it another go, and I eventually got it working. You
do need to cut the trace connecting pins 57 and 58 on the NES side of the adapter, otherwise MMC5 games will not work correctly. I verified this using L'Emperur (US). I haven't fully verified that expansion audio works, since I don't have a NES ED nor any NES carts wired to send expansion audio, but once I get something to test that with, I'll report back.
Lastly, some tips for soldering this up:
- Use a ton of flux. That helped me a lot with the jumper/bodge wires to the edge connectors.
- Use kapton or electrical tape to tape off the edge connectors, leaving only a small work area at the very top. You don't want to get solder all over the pins. This is bad, for a variety of reasons.
- Run the jumper/bodge wires underneath the resistors. This will get you the shortest wire lengths to the edge connectors, and requires you to cut away less of the plastic shell to get the adapter closed back up.
Have fun, and thanks for all of the helpful information in this thread!