Based on the bell bearings page, these are the items offered after turning in the bell to the twin maiden husks:
https://eldenring.wiki.fextralife.com/Bell+Bearings

Gold-Pickled Fowl Foot
Fan Daggers
Margit's Shackle
Grace Mimic
Glass Shard
Furlcalling Finger Remedy
Festering Bloody Finger
Stonesword Key
Missionary Cookbook (2)
Parrying Dagger
Great Arrow
Ballista Bolt
Horse Crest Wooden Shield
Sacrificial Twig

I don't suppose this can be obtained after Shaded Castle, can it?

do you have to kill him for it

Patches' Bell Bearing can be used to can be used to can be used to

***********spoilers***************For those who killed Rykard before encountering the members of Volcano Manor, effectively breaking the questline: I found Patches Bell Bearing on the ground near the tower that contains the Pulley Bow. Close to the location with the Glowing Stones. (Note, I had already encountered him there and had him kick me over the ledge.)

If you kill him after completing his quest, do the twins offer unlimited fan daggers or no?

Does anyone know if the maiden sells an infinite amount of items? I’m trying to find an easy source of gold pickled fowl foot for farmings and when speaking to patches himself, he only lets me buy 3 total

I'm a fair way through the game, with only 2 Shardbearers left (Rykard dead). I have not seen patches in game, but I went to Murkbarrow Cave as I was trying to explore as much as possible before I finish the game.

I don't know what part of my progress ruined Patches, but after clearing out the cave I did not find him there, just his items on the ground.

If you want to progress his storyline I'm assuming you'll probably have to head there fairly early on.

accidentally killed him. I really wanted his gestures, too!