Quote from: phosphor1897 on January 01, 2019, 10:15:08 AM

What happens? Does the "longest chain win" and thus all communities, except that which had the longest chain somehow, are now considered untrue -- effectively reversing the transactions of all communities that reunify except the one that happened to have the longest chain?

By default, that happens expect it's not "longest chain win", but "chain with biggest proof-of-work/hash-rate"

Quote from: phosphor1897 on January 01, 2019, 10:15:08 AM

Is there a cryptocurrency that is better suited to this type of fragmentation->reunification series of events, or does bitcoin handle this?

That depends on your perspective, but other cryptocurrency have different solutions such as :
1. On some PoS-based cryptocurrency, block made by address/node with biggest coins is considered as "longest chain"
2. Few cryptocurrency use validator who determine which transaction/block is valid

Quote from: phosphor1897 on January 01, 2019, 10:15:08 AM

Another way to put it is how would these disparate bitcoin merge multiple histories and provide a managed way to deal with those "merge conflicts" ?

Solving merge conflict is impossible without human involvement, which makes decentralization pretty much useless. Additionally, you would mess up with blocks hash and re-computation is required.

Quote from: phosphor1897 on January 01, 2019, 10:15:08 AM

It seems to me that once fragmented, nodes should never re-unify, and should instead trade against each other on some market. i.e. BTC-CommunityX and BTC-CommunityY would have to both exist independently... but what if you're not an expert at blockchain and you're just the community member standing up all these services and bitcoin just happens to be the solution you went for, and suddenly the network reunifies, all your history is suddenly lost?

Yes, i also agree it's the best solution.

But there are cheap way to prevent lose history when connected to nodes with "chain with biggest proof-of-work/hash-rate" and it's called checkpoint. Basically it makes a node reject block/chain which don't have same block hash on specific block height.
So if at least one people have enough knowledge to read documentation, source-code and made minor modification, then the problem is solved (since Bitcoin used checkpoint feature)