Plex is perfectly capable to handle mp3 files in a music library. No need to rename them for that.
On what platforms do you experience the lack of controls for videos in photo libraries? I’ve just checked this on Plex Web and Plex for iOS and they seem to be showing/playing basically as in any regular video library
As for your other request… there’s already existing requests to improve the handling of videos in a photo library as well as mixed-media libraries. I suggest you comment/vote in those to avoid distracting/dispersing votes (hence the request to search before posting duplicate suggestions).
