How would I have destroyed these photodiodes?
I'm not convinced you've destroyed them, especially if that is the 'after' photo. It's fairly hard to invisibly destroy most semiconductors.
The zero current probably was because you need to be on a uA range.
I assume you carefully flipped them to the correct polarity.
As to what is or was actually wrong (you may have introduced new issues), there is a very fried looking ceramic capacitor just above the PDs. Keep in mind that internal diagnostics have their limits. The trace which appears to go to a ferrite bead may also have an open caused by overheating but it's hard to tell from the photo.
My initial diagnosis would be that the cap shorted, causing it to overheat, trace may have overheated and become open/intermittent and the ferrite may also be damaged. The ferrite bead + large value ceramic capacitor could be a filter for the common photodiode bias supply.