What would happen if a private fork several hundred blocks ahead tried to merge back?
One possibility, as hinted by Murch, would be to pre-build a long chain, then using an attack on the hash take a few hours to construct a block with exactly a given hash. This may require waiting until an exceptionally vulnerable block appears but that doesn't really matter.


I don't see where I hinted that, but that would definitely not work. 


  few hours to construct a block with exactly a given hash.


Block hashes are SHA-256d hashes. The space of 2^256 is too large to brute force for the foreseeable future. Definitely its impossible to do in "a few hours".


  exceptionally vulnerable block


What's that supposed to be? Blocks are not more or less vulnerable.


  pre-build a long chain


The whole chain would become invalid if you changed the first element. Each block builds on the block hash of its predecessor. Therefore, two blockchains cannot be arbitrarily appended to each other.

And as already described in the comments above: 


  high number of lower difficulty blocks


It's not the count of blocks that counts but the total "weight" measured in summed difficulty.