Learn Spanish (Latin America): Apenas terminó el simulacro, tuvimos que volver al edificio. - As soon as the drill ended, we had to return to the building.

Yes, it can! However, when apenas is used as a time connector introducing a clause, it translates to "as soon as" or "the moment that." Here, it connects the drill ending directly to the action of returning.

Why is terminó in the indicative mood instead of the subjunctive?

We use the indicative (here, the past tense terminó) because the drill ending is a completed fact that happened in the past. You only use the subjunctive after apenas when you are talking about an action that hasn't happened yet (for example, "We will go back as soon as the drill ends" would use apenas termine).

Why use the preterite tuvimos que instead of the imperfect teníamos que?

Using the preterite tuvimos que means that not only did you have the obligation to return to the building, but you actually completed that action right then. The imperfect teníamos que would just describe an ongoing past obligation (meaning "we had to go back" or "we were supposed to go back") without confirming if you actually did it.

Could I use en cuanto or tan pronto como instead of apenas?

Absolutely. Both en cuanto and tan pronto como also mean "as soon as" and follow the exact same grammar rules as apenas. They take the indicative for completed past actions and the subjunctive for pending future actions.