Well, NiMH cells are now available in >2AH AA, although sometimes the rating is exaggerated when you get much above 2AH, and a common complaint is that the types which high AH capacity have high self-discharge rates.
You can't parallel NiMH/NiCd in any practical way except in large industrial strings with some fairly advanced circuitry. Been through this. There is no practical way on a small scale.
The "memory effect"- that if you don't run it down all the way before recharging, the capacity is reduced on all later cycles- often cited as a problem with NiCd, it never really existed.
NiCd/NiMH can be run all the way down to 0V without causing significant harm to the cells. However, REVERSING a cell will most definitely permanently damage a cell. Unfortunately, that's what happens if you have a multi-cell battery and run it all the way down. One cell in a 6v pack will be lower than the others, reach 0v first, the pack still develops 4.8v and you keep using the drill. Several amps is forced through a 0v cell the opposite direction as charging, and it'll actually show a negative voltage when taken out. Permanent damage.
The common method of cell death is separator failure. "zapping" a cell breaks the dendrite or whatever is pushing through the broken separator, but it's quite short term since it does not actually repair the separator- the cell will fail again in a few cycles or a few days.
In short, they're dead. Just accept it.