Correct me if I'm wrong, but this is what I got from the confusing flashbacks we were given:
She swapped her brother with Credence.
Since Credence ends up in the US as seen in the first Fantastic Beasts movie, we can assume that after the boat sank, Leta indeed ended up in the US.
Therefore, why did Leta even end up at Hogwarts when she has absolutely no attachment to the UK?
The implication appears to be that the ship set off from the UK to America, as most would have done at that time. They then must have gone to America, to drop Credence off, once Irma realised she no longer had Corvus. Which we know she did do considering Irma's name was on the adoption papers.
CREDENCE I’m sorry. Your name is on my adoption paper. Does this make sense? You gave me to Mrs. Barebone in New York.
Fantasitc Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 56
After that they no longer needed to stay in hiding, as Corvus was dead and that was the whole reason for going, they likely returned back to the UK.
KAMA (to LETA) Realizing that Mustafa Kama’s son had sworn revenge, your father sought to hide you where I couldn’t find you. So he confided you to his servant, who boarded a ship for America.
Fantasitc Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 104
Of course we know that Leta's father didn't love her and that she rarely returned home on the holidays, it's possible she was sent to Hogwarts just to be out the way further.
KAMA The news of her death drove my father insane. With his dying breath, my father charged me to seek revenge. (determined) Kill the person Lestrange loves best in the world . . . I thought at first it would be easy... he had only one close relative... you. But—
LETA Say it...
KAMA ...he never loved you.
Fantasitc Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 102
GRYFFINDOR GIRL 1 You know she stays here every vacation. Her family don’t actually want her home.
Fantasitc Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 67
Though Leta’s father Corvus Lestrange Sr. was French, it’s never said that she still lived with her father when she was attending Hogwarts. A Gryffindor girl says Leta’s family didn’t want her home, but it’s not said if the family being referred to is actually her father, or other family.
ANGLE ON TWO 13-YEAR-OLD GRYFFINDOR GIRLS pushing trunks.
GRYFFINDOR GIRL 1
You know she stays here every vacation. Her family don’t actually want her home.GRYFFINDOR GIRL 2
I don’t blame them, she’s so annoying. Even the name Lestrange makes me feel sick—
- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)
It’s very likely that Leta had relatives in Britain, since there were some Lestranges living in Britain even before Leta went to Hogwarts. The Minister of Magic from 1835 to 1841 was Radolphus Lestrange, proving there were Lestranges in Britain.
Radolphus Lestrange
1835 - 1841
Reactionary who attempted to close down the Department of Mysteries, which ignored him. Eventually resigned due to ill health, which was widely rumoured to be inability to cope with the strains of office.
- Ministers for Magic (Pottermore)
There were still Lestranges in Britain years later, since the family was in the Sacred Twenty-Eight, a list of the truly pure-blood British wizarding families written in the early 1930s.
In the early 1930s, a ‘Pure-Blood Directory’ was published anonymously in Britain, which listed the twenty-eight truly pure-blood families, as judged by the unknown authority who had written the book***, with ‘the aim of helping such families maintain the purity of their bloodlines’.
- Pure-Blood (Pottermore)
Considering it’s never mentioned that Leta continued living with her father while she went to school, it’s possible that Leta was sent to live with relatives in Britain. The last thing Leta says about her father’s involvement in her life is that he sent her and Corvus Jr. to America.
LETA (V.O.)
My father sent me to America, along with Corvus.
After that, the last thing we know of Leta before she later attends Hogwarts is that she’s on the lifeboat with Irma and Credence. It’s not mentioned what happens to her between then and going to Hogwarts.
EXT. LIFEBOAT—1901—NIGHT
CHILD LETA, IRMA, and BABY CREDENCE are in one boat, CREDENCE’S AUNT and BABY CORVUS in another.
- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)
It’s possible that either Corvus Sr. or Irma decided to send Leta to live with relatives in Britain, rather than bringing her back to France to her father or leaving her in America with Corvus Jr. to be adopted. Her father never loved her, so he wouldn’t want her presence in America to be a lead for Kama to find Corvus Jr. but mightn’t want to keep her either.
KAMA
The news of her death drove my father insane. With his dying breath, my father charged me to seek revenge.
(determined) Kill the person Lestrange loves best in the world . . . I thought at first it would be easy . . . he had only one close relative . . . you. But—LETA
Say it . . .KAMA
. . . he never loved you.
- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)
It’s possible that Leta was sent to Britain, and then attended Hogwarts because that’d be the wizarding school that’s there.
