Thanks everyone! I did talk to my council.
We landed on keeping her in her Troop and enrolling her in our Chartered Organization’s Venture Crew. I am an Associate Advisor in the Crew.
For those who are interested in the details, these are the concerns that were brought up, and details that were clarified:
The Eagle Extension seems to be tied to the council, and there were concerns that it would not be transferrable, or at least that it was unclear how to transfer the Eagle Extension.
Dual registrations are possible ONLY in different councils, not different Troops within a council, with the caveat that you end up creating a second ID for the Scout and ONLY one ID can have the Scout’s advancement, so it would require coordination to allow the advancement approval from the other Troop - but doable and making sure the two IDs don’t get merged.
Because she is already 18, she couldn’t have a youth registration in our Troop and would have to have an adult registration - some confusion about this, but it’s just different than extending her position in her original Troop and seemed like not worth trying to figure out.
Giving her any adult position in our Troop would negate her youth position in the original Troop and her ability to advance (As @AprilMcMillan1 mentioned above)
I think that was all that we uncovered, perhaps navigable, but not worth the effort, when we can simply have her in our Venture Crew and pursuing Scouts BSA with us that way, while preserving her ability to earn Eagle in her previous Troop.
Thanks again for all the help trying to sort this out.