In all likelihood the different error codes mean something like:
•Cell not reaching it’s fully charged state within the prescribed time (symptom of old cells)
•Cell is overly-discharged (If it was drained to preserve the backup caches and not recharged immediately)
•Cell is too hot/cold
•Cell did not meet the prescribed capacity requirements upon conditioning (symptom of old cells)
Since they're only spot welds, drilling them out doesn't require going very deep into the body. No more than a millimetre in fact. So the body will be damaged, but only on the inside of the unit where it won't be visible after reassembly.
Hope that helps!
It took me about 48 hours allowing two coats of conductive paint to fully dry; maybe two hours actually working on it, figuring everything out as I went along.