If I activate Tactical Retreat in response to my opponent's Falling Star and they then respond with Abandon, after the Abandon resolves, can I then activate Tactical Retreat again assuming I have the runes to do so before the initial Falling Star kills my units?

Yes, you can activate Tactical Retreat again in that situation.

Because Tactical Retreat is a [Reaction] spell, it can be played whenever you have priority, including when other spells are already waiting on the chain to resolve.

Sequence of Events

1. Initial Play: Your opponent plays Falling Star.
2. Your Response: You play your first Tactical Retreat targeting your unit.
3. Opponent's Response: Your opponent plays Abandon targeting your Tactical Retreat.
4. Chain Resolution (LIFO):
   • Abandon resolves first, countering your first Tactical Retreat and returning it to your hand.
   • Falling Star remains on the chain, waiting to resolve.
5. New Opportunity: Because the chain still exists (containing the unresolved Falling Star), you have an opportunity to play Tactical Retreat again from your hand, targeting your unit.
6. Resolution: Your new Tactical Retreat is placed on top of the chain. When the chain resolves, your second Tactical Retreat resolves first, creating the replacement effect to protect your unit, followed by the Falling Star.

Important Notes

• Replacement Effect: When your second Tactical Retreat resolves, it creates a replacement effect. The next time your unit would die this turn, the effect will intercede to heal, exhaust, and recall it to your base.
• Costs: You must be able to pay the cost for the second Tactical Retreat (2 energy) to play it.

Note: I could not fully verify this ruling against the retrieved rules. Specifically, the exact effect of 'Abandon' and the timing mechanics regarding priority windows during chain resolution are not fully supported by the provided context.