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.
"Mm, she's not exactly the same doorstop," he explains. "She found the resurrected one and they got back together. Now she wants to know if you wanna do the same thing with Nathan."
He presses his hands together, pulls them apart. "They split..." together again, "they merge."
"...Well, there's a thought, but while I'd welcome being a vampire again and miss Kerron quite a bit and have no strong objections to resuming being stuck exclusively on you, I don't think Nathan's going to want to share an identity with a version of him who -" Airquotes! - "'cheated on you'."
[Hi, Nathan, I'm Shell Bell, I don't think we've met. Have you been all right here? Why don't you think your liveling would want to be you?]
[Hullo, Shell Bell, I think he'd be torn to pieces over the fact that I've spent the last century comfortably enough, unmatebonded, enjoying myself with miscellaneous ladies of Downside instead of remaining loyal to his mate in spite of the fact that he'd see red if he knew I'd touched his mate.]
[...That seems like an obviously silly thing to think. You two could customize the merge, you know, I'm mostly Bell and not Shell. Would he mind not thinking like that anymore?]
[Not sure. I don't know if that would work, anyway, mate bonds are impervious to magic.]
[Obviously not completely,] Shell Bell points out. [You had one and now you don't. Maybe there's room around the edges to make yours - his - work more like Alice's does.]
[I'd like that,] the Joker contributes. [Seems like the least awkward way to get to love both of him.]
[Put some clothes on, babe. Shell Bell, you could just open the door to here even after you fused?]
[If I don't think about it, it's Atlantis, but I concentrated and I opened the door to my Downside apartment without too much trouble. You'll be able to visit friends of yours here through Milliways. And the place isn't going to be like it is, anyway, me and the other Bells are going to destroy that sadistic ruinous setup.]