Preterm labor at 28 weeks — tocolysis choice, corticosteroid timing, and magnesium for neuroprotection

A 26-year-old G1P0 at 28 weeks presents with regular uterine contractions every 5 minutes, cervical dilation of 2 cm, and intact membranes. How long can we safely tocolyze with nifedipine or atosiban? What is the window for betamethasone to be effective for fetal lung maturity? When is IV MgSO4 indicated for fetal neuroprotection in preterm labor, and what are the dosing guidelines (ACOG/RCOG)?