There is not other block-chain with bigger combined work amount, so checkpoints in Bitcoin-qt are correct ones
How do you know that there is no other block-chain with bigger work amount?
Perhaps fork started right from genesis block, and was mined all the time at private cluster at super high-speeds (now on cluster of asics).
Bitcoin is probabilistic. The scenario you mentioned does not make much sense, because nobody would waste his resources in that way. There are other ways to break consensus, namely through Alt-Coins. If you have developed a far superior mechanism people would flock to that.