What are the odds that the Bitcoin network will suffer a freak alignment of connections where big cluster of nodes will clump up remaining separated from each other for prolonged periods before eventually dissolving back into a more homogeneous foam?

I disconnect all the time. It's not really a problem.

You're talking about allopatric genetic drift (see left column),


The longest chain of the two after being merged becomes the main one.

It's pretty unlikely to happen for long enough to become severe. Every peer in one segment must lose their connection to the other segment. Even if one ISP gets cut off from the Internet, it's likely that at least one person on the cut-off ISP is multihomed, and they will relay transactions and blocks between the networks.

Bitcoin makes outgoing connections to 8 random peers in different /16 networks, so I doubt the network would "drift apart" by itself. Especially when everyone is connected to IRC.