With regards to the volume issue I can only surmise that somehow that setting was in fact enabled but it was actually displaying as off, that would give you the impression everything was fixed at max when in fact it was still variable and would nicely explain why the sound level was rising and falling.
I’ve observed other settings in LDS that when altered don’t seem to display what was initially set, but closing LDS and restarting seemed to correct that behaviour. But it’s only a guess mind you it’s happened in your case, anyhow, let’s trust your “Reset” has sorted it all out.
I’ve never had issue on rebuilding library but my content is on internal ssd and WiFi is quite strong and fast, coupled with only having about 650 albums, it all processes within a minute or so.
I’ve read when undertaking any tasks with the library you just have to give it time to complete and avoid closing LDS, this allows all background processes to complete.
Can’t say I’ve experienced any freezing but if the device is hard at work processing a new library it may well appear unresponsive, again I believe you just have to give it time and certainly avoid interrupting that task by turning it off, that could just introduce potential errors if the system can’t recovery correctly.
Regarding metadata, I would expect if files are edited with reliable software the program employed would take care of what’s acceptable or not, I use Metadatics for macOS and it’s actually corrected some missing data left by programs such as Fission. Most editors also allow you to strip out data not required, which I do as a matter of course.
I don’t have experience of content stored over a network so can’t really add much on that topic, but I’m sure others can advise on what to look out for and probably how to improve the overall process.