Shell's memories are going to be perfectly accessible, but they will not come up automatically in response to stimuli unless the resulting Shell Bell is expressly open to it. They will keep Shell's hard-earned tolerance to pain, insofar as that's separable (Shell is quite sure that she'd be able to make a pentagon if another situation as important as getting ahold of Sherlock to mint Juliet came up). All of Shell's insights about Sherlock's importance stay. Her itchy discomfort with Sherlock not getting her importance will be folded into Bell's resignation to same. The merged Shell Bell will be comfortable referring to either past in the first or third person, but will default to using "I" for Bell and "she" for Shell. She'll torch, but that's the only supernatural property Shell's bringing to the merger; she'll retain all Bell's magic. Shell looks a little older, but not much; they'll go with that, as they think torching probably conflicts with actually aging and they don't think twenty is the ideal place to sit forever and whatever Shell is supposed to be is closer. They are omitting the nightmares, good riddance.
"Last minute questions, comments, reasons to expect this to be a bad idea in some way?" Bell asks.
Shell shakes her head. She is looking forward to not being a separate person anymore.
"I'd like the company, but I don't need it," says Shell Bell, "Elspeth gets doors very frequently and I won't have to stay much longer than I feel like."
"...Hello?" says Elspeth, who found the door in the first place. "What's going on?"
Shell Bell kisses Sherlock's cheek and steps through the door to Aurum. "Hi, Elspeth, we haven't met, I'm Shell Bell. There's a glitch in resurrection-with-evils called Downside. I need to talk to your mom, and the Joker and this extra Nathan need to talk to the Nathan here."
"Probably best if Kerron's not there to witness the explanation, and possible attempts by Nathan to relieve me of the burden of intact bones," adds Timer.
"Mama - and Nathan - are both in the throne room. Rosalie has all the wee ones for the next while," says Elspeth, "Kerron included."
"Hello," says Golden, not particularly surprised - presumably Elspeth notified her of the arrivals. "What's all this, then?"
"You know," says Timer, "I held out hope that you'd just think I was a garden variety alt till somebody could explain, but I suppose arriving arm-in-arm with the Joker without him even singing soprano first torched that interpretation."
"Who in the name of Maughold's left asscheek are you supposed to be?"
"Call me Timer," says Timer.
"All this," says Shell Bell, "is a glitch in resurrection-with-evils. There is an afterlife, it is shit, and it does not let people who arrive there go, even if you pull on them very hard - you get forks, instead. I folded back into myself. I'm both."
"I died," the Joker adds, "but I'm fine, sweetie, look," and he taps a pentagon from Timer and teleports directly into a hug with Nathan.
"It's not bad always for everyone," Timer volunteers. "I was fine. But then, Shell Bell would've been fine if her dead version had kept her magic. I had mine. Kept me out of the way of torturers and - such."
"Torturers? That's a class of people you need magic to avoid? Oh, for the love of - even ordinary worlds need serious repairs, Shell Bell's was worse than most, and she comes in saying it's shit, I'm inclined to believe her, but -"
Extra snuggles for Nathan.