Can the blockchain be outpaced by a chain of low-difficulty blocks?
There are two problems with this:


The "longest" block chain is selected not by total number of blocks, but by total difficulty.  A chain with a large number of low-difficulty blocks would not win.
The Bitcoin reference client hard-codes the hashes of a relatively recent block as a "checkpoint" and will reject any chain not containing that block at the correct height.  So any chain that diverges before the checkpoint will be ignored.