I noticed that there's some corrosion/burn on the HV battery ECU where the Wire Frame No 2 connects. All other parts of the ECU are pristine. Do I need to buy a new ECU, or is it possible to salvage this one?
I'd certainly be trying to clean it with some CRC contact cleaner or something blast that sucker out and see what you got after you get a lot of that pins all look like they're there.
We've been trying to track down replacement sockets so you can solder a new one to the board... This is such a common problem that anyone who has rebuilt packs for years has a whole box of ECUs that look just like this. Someday when we track down a replacement plug we'll have lots of spare ECUs. As for yours, it looks like you got some missing pins. One of my friends who lost a pin soldered a wire to what remained of the pin on the inside and bypassed it from the socket and crimped it direct to wire frame. Alternatively you can get a used one on Ebay for $70 to $100.
Unless you have mad skills with miniature soldering repair, just buy a new wire frame #2 for about $65 and a replacement ecu from ebay for $100 and call it a day. You don't want to be messing with this 5 times a year. Was this battery sitting unused for a while? If so, you likely will need 3 or 4 modules also.