Can separate Bitcoin chains be merged back? After a fork in a given chain, is it possible to merge the two chains back together so that they share the same blockchain?
No it's not, it must be a blockchain not a blockgraph, if two blocks share the same parent then only one of them can be in the chain, and because blocks can't be 'editted' without simply remining them, you have to choose either one chain or the other. Even if you could, do that, there's a host of other issues like the same coins being spent on both chains, or the block height in the coinbase being wrong