Ella Gross is the only known trainee, so her insta posts are probably the best source of evidence.
Is she training: Yes.
Ella posted a dance practice video (last one) to her insta recently (which is unusual).
Is she training with The Black Label (but in the US): Yes.
She reps TBL on a custom jersey in her most recent post (coincidentally, right after the dance practice one).
Is she good enough to debut?: Close (for the group maknae)
Her dance is solid, but still a little simplistic. Could be good enough late next year.
Is she old enough yet? No.
but she'll be 14 all next year, and 15 past December 1, which is normal for the maknae. Minzy was 15 at debut.
Is TBL currently preparing for a debut? Yes.
They did a lot a hiring over the summer. Far more than you would expect for just their current artists: Somi, Loren and ZionT.
They also talked about raising more money to expand their acts.
If she goes dark for a month and/or visits Korea for at least a month (she did for less time this year) then debut could be very close.
(Edit) and this:
(Edit) and this promotional piece for Teddy and TBL on MAMA while Somi was the host:
They had this all planned out.
"One more member" implies that they could release by the end of next year. It is also consistent with Ella being one of the already selected members.