Learn Spanish (Latin America): Encontramos un baúl viejo en el ático.

Yes! For regular -ar verbs like encontrar, the nosotros form is exactly the same in both the present tense (encontramos = we find) and the preterite past tense (encontramos = we found). You have to rely on context to know which one is meant.

Why does the word baúl have an accent mark on the U?

The accent mark forces a "hiatus," meaning it breaks the word into two distinct syllables: ba-úl. Without the accent, the strong vowel (A) and weak vowel (U) would normally blend into a single syllable (a diphthong), which would change how the word is pronounced.

Could I say un viejo baúl instead of un baúl viejo?

You could, but it changes the nuance. Placing the adjective after the noun (un baúl viejo) is the standard way to state a physical, objective fact—the trunk is old in years. Placing it before the noun (un viejo baúl) gives it a more poetic or emotional tone, emphasizing the idea or feeling of its age.

Could we use adentro del ático instead of en el ático?

Yes, adentro del ático (inside the attic) works and is grammatically correct. However, en is the most natural and common preposition to simply say something is located "in," "on," or "at" a general location without over-emphasizing the "inside" aspect.