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IMHO a request system is sufficient. Remember GPL comes from a time where Internet was not granted and magazines with floppy disks were a thing.

Unlike the full GPL, the LGPL doesn't require applications of the library be licensed under the same. It would apply only to improvements to the library itself.

If you ship the library, the requirements for the library itself are very similar to the GPL.

The difference you describe mostly applies to system libraries (those libraries that are not distributed alongside applications).

No, the main requirement is that they link to libfprint dynamically or provide object files such that effectively you can replace libfprint with a different version of libfprint of your choosing, but there's no requirement to open source the work that makes use of libfprint.