404(B) evidence goes to Alex's motive to commit the murders because Alex was on trial.
If Becky was charged with any crimes, then her motive would certainly be at play. But that's not the issue here.
The real issue here is whether Becky (or anyone) improperly influenced the jurors. Becky's motive, actions, book sales, egg lady love, podcasts, love of Lady Gamecock basketball, preference for mustard-based barbecue, etc. - none of that matters. What matters is what influence - if any- her actions had on the jurors.
What matters is what was going on in the jurors' heads. What matters is what outside events impacted their verdict.
Judge Toal is keeping a tight shot group in this hearing.