Today, after eight months, she does.
She makes sure that Sherlock and Tony are both awake, finds that they are but do not want to join her in the bar today unless she finds something interesting, checks her coin supply and finds it sufficient, and steps through.
"Okay." Sip sip. "I want to go check the Belltower in case there's anyone there, but then I'll be happy to go back to your world - unless you're waiting for something or someone here?"
"Nope! Just hanging out with my best bud," she says, patting the hammer at her side.
Shell Bell finishes her shake, teleports up, finds the tower empty and nothing new in the book, and teleports back down. "Shall we?"
"Sif!" says Darcy, waving. "This is Shell Bell, I found her in Milliways, she says she can maybe fix the Bifrost, do you want to hold the door for us so we can go talk to Heimdall about it?"
"You have the most interesting friends," Sif says dryly, coming to take the door from them.
(She has quite forgotten that she illusioned up a crown - she'll remember if reminded, of course, but it's slipped from her attention without the weight on her head to remind her.)
"Thanks," says Darcy, and she turns to Bell. "Can you take us straight to the end of the bridge?"
"Sorry—there's only one end that's the end," she explains. "The other end is the beginning."
There is a man in elaborate golden armour standing on the very end, gazing out into eternity.
"Heimdall!" says Darcy. "This is Shell Bell."
"I heard," says Heimdall. "Can you repair the Bifrost, Shell Bell?"
"You can talk to Heimdall," says Darcy. "He's cool."
"Okay. I can certainly try to repair the Bifrost! There's not, like, more than one way for it to be 'fixed', is there? If there is then I have to know what I'm aiming at or my wishes get confused."
"It stood here once," he says, gesturing to the empty space past the broken end of the crystal on which they stand. "Unchanging, for many thousands of years. You speak of a magic that is not known to me, but if it needs only a clear example, it will not lack one."
"It's good at following examples," says Shell Bell. She tries a pentagon, but it's not heavy-duty enough. She tries a hexagon. There it goes. "All better!"
"Will Sif mind waiting with the door while I go see if your Tony here is worth talking to? I barely ever find a door, if she lets it close I might wait a year and a half before I could go home."
"Okay," says Shell Bell. Teleportation occurs! "Do you want me to put you on the brainphone so you can let me know from here if you get sick of standing in one place?"
[Like so,] says Shell Bell. And she turns invisible and teleports to wherever Tony Stark might be on this world.
That's a weird thing in his chest. She wonders what it is.
Also, he doesn't seem really prepared to receive visitors. She hops to just outside the house, and attempts to add the local Jarvis to the brainphone; this'll simply fail if there isn't one, but she thinks her world is the odd one out there.