Yeah that's what I did, I can load up the save files just fine but the gallery itself was still locked. I'll try out this file soon and see what happens
Haven't played it yet, but there's a good chance you would have to first "unlock" the CG and animation and whatever in the "extra" in the main menu, which would need you to have a game over or a scene, maybe? At least that's one thing to consider if things are similar to the first game (or the circle's other games for that matter).
EDIT: that said, I just tried it out (downloaded from the link in the thread, installed, then used the first shared full save from post #92) and it...
works.
Full gallery unlock. So, keep trying, you were doing something wrong. If you don't get asked whether to overwrite files when copying the full save files from the archive into the game's save location - you are doing it wrong. And obviously choose 'Overwrite All'.