What would happen if a private fork several hundred blocks ahead tried to merge back? Suppose mining pool has been off the main network for a long time so that they are 100 or so blocks behind, but managed to mine 200 blocks in the period (say, some combination of had 33% before, deliberately selecting easy blocks to extend, and ...).

Obviously, this can happen by choking down the difficulty, but I'd assume there are protections against such an obvious attack as that one. Update: assumption verified; reducing the difficulty doesn't work here as expected.

I found two roughly similar questions, but neither seems right:

Can a double spend attack be obfuscated by mimicking the transactions seen before the fork?

Countrywide Internet isolation, inevitable fork

The obvious answer, if it contained only mined transactions, would be to manually discard it, but I don't think that assumption would prove valid.

I am bothered by the belief that this is a duplicate of longest chain vs. highest difficulty. Marking this as duplicate pre-supposes an entire class of cryptographic attacks on hashes does not exist, and many other things as well.

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 finally guessed what a no-malice version might look like and I found this: Scenario: disaster splits the Internet into dozens unconnected fragments Basically, bad things happen if the largest connected network is &lt;&lt; 50%.
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.