It's worth noting that if someone can't be contacted, the maintainers aren't out of luck yet. If the contribution is deleted (and then possibly reimplemented later by someone with whom the project _is_ in contact) then the issue is resolved. It can be a lot of work, though, depending on the size and importance of the contribution, and reimplementing the code in a way that doesn't derive from the original submission can be difficult or ambiguous.
GPLv2 requires you to either distribute the source with the binaries, or provide the recipient information on how to obtain the source code. It also states that providing a link to the source code next to the binary download on your website is sufficient. See GPLv2 section 3 for more details.
TikTok have not provided a link to its source code, not are there instructions on their site or within the download package indicating where users can obtain the source code. Therefore, it's a violation.
Paying for a license would be nearly impossible, as the OBS team would need every contributor to sign a CLA to give the OBS team the rights to relicense/dual-license the OBS code base.
OBS uses FFmpeg for media source playback, some audio encoders, and specific video output contexts. Usually FFmpeg isn't used for video encoding, though -- that's handled by either a direct hardware encoder implementation or by a native x264 encoder (i.e. not FFmpeg's x264 implementation). OBS supports FFmpeg output in Advanced mode, but it's by no means the default.