Gregte, sounds to me like it wasn't too-high current that destroyed your NiMH cells. They did fine for the first 30 minutes, despite high current.
But then they ran out of power - they had exhausted the 12Ah they had available, while the 16Ah Lithium cells still had more power to give. The lithium cells kept pumping current through the DEAD NiMH cells, which is an EXTREMELY bad thing to do to any rechargeable cell.
When any cell discharges, it puts out current from the + terminal and takes it back in through the - terminal. When you charge the cell, you force current the opposite way - pushing current into the + terminal and getting it back out through the - terminal. Those are normal modes of operation, and will not hurt the cell.
When a cell is completely exhausted, it won't push any current anywhere, of course. But you can charge it by using an external device (battery charger) to push current into the + terminal, as I said, and you won't harm it. You'll just charge it.
But if you have a completely exhausted cell, and try to push current into the - terminal, you will ruin it VERY quickly, like in seconds. And that's what your lithium cells did to your NiMH cells.
That's why it's so important to "balance" your battery. A battery is made up of a number of individual cells. And there are always slight differences between cells - they are never exactly identical. If one of them gets exhausted before the others go dead, the others will keep pumping current through the exhausted cell, the wrong way... and wreck it almost instantly.
