Fix various namespace prefix conflict resolution bugs and namespace shift bugs

There are two linked issues:

- Conflicts couldn't be resolved by changing the prefix name.
- Lacki

Fix various namespace prefix conflict resolution bugs and namespace shift bugs

There are two linked issues:

- Conflicts couldn't be resolved by changing the prefix name.
- Lacking a prefix would shift the namespace as the default namespace,
causing elements to suddenly become part of the namespace instead of
the attributes.

The output could still be improved by removing redundant namespace
declarations, but that's another issue. At least the output is
correct now.

Closes GH-11777.

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