My best guess at how Ro returns:
- She is revealed in the last few minutes of this week's episode, but the long talk between her and Picard doesn't happen until next week.
- She's actually either the sole one or part of the group that "brings in the cavalry" and rescues the Titan (has to be, otherwise she wouldn't be introduced in the last act)
- Most speculatively - she knows Vadic, personally.