What happens if star crossed the nonambushed unit in reaction to ambush while I’m the defender

Yes — but it won't save the battlefield: Star-Crossed resolves first (it's on top of the chain) and returns the attacker's pre-Ambush unit to their hand, yet the Ambush unit was already on the battlefield and stays there. Because that unit is still present, the attacker keeps their presence at the location and keeps the Attacker designation — control isn't lost while chain items are pending, and the Ambush unit alone is enough to hold the attack (mirror of FAQ #10057).

Two important catches:

• Timing: you can't react to the Ambush play itself — units finalize and enter immediately with no window (FAQ #9978). You're actually reacting to the window created by the Ambush unit's enter/attack triggers (or another chain item), not to the play.
• Cost: Star-Crossed requires both a friendly and an enemy unit, so you must also return one of your own units. If that was your only unit at that battlefield, you're left with zero units while the attacker's Ambush unit remains — when the combat resolves, the attacker wins and establishes control, Conquering the battlefield (Rule 466).

So the only way this plays out well for you as the defender is if you still have another unit (or a play that replaces the bounced one) to keep defenders present.