On notes and ghosts: My hunch is that these never expire, so I don't think they're a good indicator of how much activity is in a particular area of the game.
On the Beckoning Bell: This is supposed to summon players who have already used their Small Resonant Bell, but according to the wiki they have to be standing near where you use the Beckoning Bell (if they're at the boss gate and you're at the lantern, you won't find them). I don't know how many people are still doing co-op.
On the Sinister Resonant Bell: In most areas of the game, you can only invade players who have at least one co-op partner, or who are also ringing a Sinister Resonant Bell, AND who are in an area where the boss is alive. If you're in an area where the boss is dead, other players can't invade you even if you're ringing the Sinister Bell. This significantly limits invasions, especially since as mentioned I'm not sure how much co-op is happening. There are a few areas later in the game where a Chime Maiden ringing a sinister bell will spawn naturally, and you can invade players who haven't found and killed her, but they aren't typically hard to find so this is a pretty narrow window.
In addition, there are level restrictions on both co-op and PvP, so even if other people are trying to be summoned or trying to invade, you might not be able to match with them.
The wiki also says you might need to restart your console if you resumed the game from Rest Mode, I don't know if that's true but it's worth a shot.
If you want to set up a time to try multiplayer, I'd be willing to give it a shot. I might need a bit to get a character into the right level range, depending on where and what level you are.