What is the realism of internet fragmentation at scale in 1020 years?
1020 years, really?

You should not be spending your short life discussing what will happen in a thousand years. Humans generally can't even make accurate 10 year predictions.
Can the current set of consensus rules on Bitcoin survive long network partitions without significant forks?
Yes, the mechanism for resolving the problem that you wrote about is deeply built into the protocol. The longest chain wins, even if it sometimes takes a while to resolve.
Have you tested Bitcoin in low-connectivity or partitioned environments?
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Works fine.
How would it be most practical to maintain Bitcoin as it is worldwide when the internet ceases to be worldwide?
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It won't be practical but it will be possible. I doubt that internet will cease to be worldwide, countries would lose too much economically by doing this.