so we've obviously seen the weird guy at the end of that bridge pointing at the tree and he won't talk to you. after the guy who wears the Charcuterie board as a necklace disappears from the Round Table, you are supposed to know that hes on some random sidewalk in Altus Plateau. You talk to him a few times. then, you are supposed to know he goes to the bridge where gold mask is. then you get a bunch of juicy details. then you are supposed to magically know that he and Gold Mask take the minivan all the way to a remote bridge in Mountaintops of Giants! and you get some more juicy deets. THEN, you are supposed to magically travel BACK to the capital, but before you finish Crumbling Azura, and then skedaddle up to that Coliseum you cant enter, run to the back of it, AND HEY! THEY ARE BOTH CHILLIN! and Charcuterie Dude is begging you to figure out why Gold Mask is moving his fingers the way he is...
So now, you have to magically figure out to run to a certain point in the Capital, use an Incantation I thankfully had bc I was in NG4 but dont even recall remembering where I got it - cast the incantation at some random statue, you get a BIG REVEAL AND THE STATUE CHANGES, then you run back to the guys, they ramble some more, then you go to Crumbling, finish crumbling, then at the Ash capital, find them both spread out with a few more dialogues and its over.
Alexander you have to pop him out of a hole, then you have to find him near the Lakes, pop him out of another hole with an oil pot, then magically figure out you have to find him in volcano manor bathing in a lava waterfall, hes in a spectacular mood... then he ventures to an extremely hard to find area in Crumbling and you fight him and he's easy and you are rewarded with one of the best Talismans in the game.

Millicent quest was actually pretty awesome. there's a nightmare boss fight in this tiny area of scarlet rot in Haligtree you have to kill the big lizard type boss and you get to complete her Questline.

Sellen will always be my favorite quest because of all the fun clothes you get. I have legitimately never even seen this Boc guy you speak of. Not sure who Diallos is.

You should do some NG+ and find the bosses you missed.