@mattkhan, BryanC, Lepa and mwillems, thank you for the support and for the comments regarding file location.
As reported earlier, this is one of the the first things I checked and all my library data is (or should be) saved on the local drive on which MC is installed (my HTPC), primarily appdata/roaming. I have never touched these options, so they are whatever the defaults are or were when I first installed MC.
I attach a screenshot of the relevant settings.
I also recorded a few videos to show how WOL works with CMC as a front end launching MC as an external player, as well as a few videos showing how things currently work (or rather don't) with MC. Apologies for the poor quality and for not editing them, but I didn't have much time.
First, here is how it works with CMC:
https://mega.nz/file/pnk3xLaT#Z-3HpPSROQm9hZ5311mRF7ZrovQ7L4W1N_9zrxEG4f4Simple, easy and user friendly. My family members have no idea what's happening under the hood, they just know they have to wait a little bit before playback starts. That's how it should be done in my opinion.
Here is how far I managed to get trying to make it work in a similar way with MC, thanks to Mattkhan and DatDude's help:
https://mega.nz/file/Z7ElyC6A#HpksIRuJFVbdqyGSKn41nqrFgjmD9qkiL-dYYHRX5HoUnless the devs extend that timeout delay to give the external WOL command time to wake up the server, and try again playing the title when MC gets the control back, nothing more can be done on the user's side.
Here are some issues that happen with MC even with both servers online, so not related to offline info:
https://mega.nz/file/UqchjY4K#q0vyzwx83YHtCIhOpAH4bxbPEUx1lSDANT93uUrsLBgHere is the behaviour with one server offline (B2K):
https://mega.nz/file/c2tU0T6b#RfEpobp0WpXIkVufStjtsOYKrHF_Vlanjgiv40HgZqQAnd finally, here is the behaviour with both servers offline:
https://mega.nz/file/lutiDTLR#azSreBiQCUvJNwnYDCzm1X8RNwu39-EDfNPYU72YXdYIt was much worse yesterday, but you can get an idea from the animation in the other video, and imagine that the cover art in the top right corner ends up entirely black. As all the needed information is displayed even with both servers offline, it proves that there is no need to access them. So I have no idea why switching the servers off causes these issues with the cover art animation or makes the interface so sluggish.
I understand that this won't change, but I just wanted to share these before moving on.
Thanks again to all those who helped, and I hope that people won't mind living with these issues, not knowing where data is stored, or why offline shares need to be accessed when there is no need to as all the info is clearly available locally in the database, in cache or on the local appdata/roaming folder.
I'm back to using CMC as a front end for now as it works better for my needs, and I'll keep using MC as an external player. It's my favorite player by far, not only because of full menu support but also thanks to all the progress made in JRVR over the last couple of years. And thanks again to Hendrik for adding HDR 3D LUT support to JRVR, I'm almost done validating the data, it's working great and I should post some information in the relevant thread soon.