The problem with that approach is that the interface puts bias on the headphone output, which will be passed on (in reverse) to the recording if I try to "fix" it.
I understand that you are saying only the headphone output is out of balance with the left side being more dominate? correct.
So, if you compensate the headphone imbalance by adjusting the mic/line LR balance, then the recorded balance (which is correct and only becomes imbalanced by you manually adjusting the pan knobs)?
If the above is true, that indicates the problem is isolated to the headphone circuitry. There is likely an internal bias adjustment that would take any TV/CB type repair shop longer to get access taking the case apart than to repair. Likely cost if that's all they find wrong will be their minimum bench charge. Certainly cheaper than replacement cost.
Just speculation on my part taken from the information listed here on the forum post but I'd suggest getting an electronics technician to look at it before disposing of it.
Charlie