So if Neb had randomly shown up at ACME, she would have been screwed. ;-) Yeay for VILE! Actually I think that was part of the point of VILE being the best place for her to have landed, because they'd be the least inclined to contact authorities.
But lets move to if an agent found her, then. What happens then?
In that case, I can't honestly say. VILE is pretty straight forward in that if Carmen lets you stay, then you're part of VILE, like everyone else in VILE. It doesn't sound that Neb showing up at ACME, or even being found by ACME, would make her part of ACME, or would even grant her the opportunity to become ACME. I have to conclude then that her showing up at ACME ultimately would have had little impact on who she became, other than being another turn in the road.
The next thing down the line would be Neb purposely showing up to join ACME as an adult, which would mean she was far more with it than she was in the original scenario. Not really the same person at that point, but lets go ahead.
How she turned out would first rely heavily on how her background is treated. If say, only Chase knew or ACME higher-ups and they decided to help her figure out her situation while keeping her safe early on, Neb would probably be much more stable than she is now. If it followed a similar path to how it went in VILE, with her situation remaining largely left on herself while ACME acted as a passive sanctuary, she'd be less stable.
Next would be events. At ACME she wouldn't have had to endure ALS or Flag, and the highly structured nature of ACME may have kept her from drifting too much. Unless it didn't. I'm not ACME, so I can only guess. That said, her time with Flag wasn't all bad, and she'd still have to deal with Seraphim.
If we look at the "more law abiding or less" she'd be more abiding, until/unless it came to a point where something she needed couldn't be obtained legally. There's a good chance she'd chafe at ACME's black/white stance on things, and VILE would never really be the enemy to her, but loyalty to her current protectors would keep her from switching sides. That said, the tower heist could change her feelings on VILE, depending on what she's told.
Who she connected with at ACME would also play a part, as well as if anyone ever learns about her...talent. I can see Neb telling Chase if they got close, but first they'd have to get close. If they did form a relationship (not a romantic one), Neb would be sturdier now. Maybe. I don't know how well she'd bond with other ACMEs because she'd probably be just as flighty as she is with other VILEs. How her existence/situation is handled would definitely affect that as well.
Overall, she'd probably stronger in some areas, but weaker in others, based on the different dynamics of the two groups. ACME's structure and outward legitimacy would help in some ways, but VILE's organic-ness and anonymity helps her in other ways. After that, it's events and personal relationships that would determine what she becomes.
In Ultramarine, like, right now, Neb is just discovering that she doesn't actually know Carmen as a person. What would be the first thing Neb does to get to know Carmen? It doesn't even need to fit into the RPs, I just want to know what Neb would ideally do.
Well, ideally it will fit into the RP, because that's what she wants to do. Ideally how she'd go about doing that (based on where we are now, like, if we were just suddenly teleported home) Neb would first give Carmen some space, because damn. But she'd be there on the fringe, because Carmen might need stuff. While doing that, Neb would probably think a lot about everything she knows about Carmen. She might write in a journal (I might write a journal!) similar to how she did with Flag.
That would probably bring up some questions. Some she'd ask Vic, like how she was on the MAMBA, but some she'd save for Carmen. She'd probably watch her some more, until she feels like maybe Carmen wouldn't mind some small talk, and go from there. They could take some walks or gets some food, or Neb would help her with shadow stuff. Maybe steal a landmark or two.
While it does seem kinda like what Neb does already, before this, Neb was in the mindset of looking to Carmen for safety. Now that she's taking that on herself, she can look to Carmen as another person, like she'll be able to do for pretty much everyone else.
First, this is tricky because Neb has essentially redone eight years of her life over, altering many problem dates in the process. That would leave events pre-Flag and post-Cadence in the ring. From that, assuming she can bring her current knowledge into the past with her, she'd redo the day she was made Carmen's shadow so she could preempt pretty much everything in Ultramarine and be a better shadow from the get-go.
That said, while Neb did change her personal timeline, she didn't realize what had happened until after Seraphim. A lot of guilt came from missing her cues from her first life, so it's also possible that she would go back to when she should have first met Carmen and be the competent cohort she was supposed to be.