The sources for mediainfo contain headers for language bindings, such as python, jni etc. These are located in MediaInfoLib/Source/MediaInfoDLL but are not installed by emerge when installing the package. The headers should be copied to /usr/include/MediaInfoDLL when installing the package.
I think you mean media-libs/libmediainfo.
Yes, correct.
The related c(++) header files are now installed for libmediainfo-0.7.45. All the other files types really don't belong in /usr/include.