On the motherboard, there is an interference filter for each serial port which can be damaged. The one for Serial B is RP2.
If you measure the resistance across the part between most pairs of pins that are physically opposed, you should see about 50 ohms. ie. pin 2 to pin 19, pin 3 to pin 18, etc. ... except for the pairs at the ends (pins 1,20 and 10,11), which are directly connected. If you have an open circuit between any pairs then that may be the point of failure.
Another possibility is that a connector contact or circuit board trace was damaged by the high current. I think only the ground pin (pin 7) would be at risk from a 5V source. You should measure close to 0 ohms between pin 7 and the metal chassis.
I measured ~51-53 ohms of resistance across all of the interference filter pin pairs, with no resistance at 1,20 and 10,11. So that seems to be okay.
On the DB-25 ports themselves, I also measured 0.1 ohms of resistance between pin 7 and the metal chassis, so that also checks out (on both Serial A and B). Out of curiosity I measured resistance between the other pins and the metal chassis (I hope that's okay?), hoping to discern any difference at all between measurements of Serial A and Serial B. On pin
Hopefully all my poking and prodding isn't making anything worse.