Placenta previa at 34 weeks with active bleeding — expectant management vs emergency caesarean?

A 35-year-old G2P1 at 34 weeks with major placenta previa presents with painless bright red vaginal bleeding of approximately 300 mL. Previous C-section scar raises concern for placenta accreta spectrum. She is hemodynamically stable and the fetus has a reactive CTG. What are the criteria for emergency vs planned cesarean in previa? How does the previous uterine scar change management? What is the role of MRI in diagnosing accreta?