Q: Has the library behaviour been modified? If so, how has it been
modified? If the library has been modified in ways that change the API or
behaviour then there may be a case for copying. Note that fixing bugs is
not grounds to copy. If the library has not been modified (ie: it can be
used verbatim in the distro) there's little chance of an exception.

A: Yes. API has been changed. See
https://github.com/minetest/minetest/issues/906 and
https://github.com/minetest/minetest/commit/8aa930f28e69f3518831500022988ca2a4b6985d#L3R50
Attempt has been made to push this changes to upstream jthread, but to no
avail. And, anyway, minetest recently dropped support for building with
system jthread altogether (planning to fork or replace it)

Q: Could we make the forked version the canonical version within Fedora?
For instance, if upstream for the library is dead, is the package we're
working on that bundles willing to make their fork a library that others
can link against?

A: No, it doesn't make sense, because changes are only usefull for minetest

Q: Are the changes useful to consumers other than the bundling application?
If so why aren't we proposing that the library be released as a fork of the
upstream library?

A: No, see above.

Q: Is upstream keeping the base library updated or are they continuously
one or more versions behind the latest upstream release?

A: Upstream using old version of library modifying it to their needs (and
intend on doing so)

Q: What is the attitude of upstream towards bundling?

A: they insist on keeping bundled version

Q: Is there a plan for unbundling the library at a later time?

A: AFAIK, yes, by replacing it with different implementation. But no
timeline given.