Learn Italian: Durante la lettura abbiamo scoperto un erede legittimo. - During the reading, we discovered a legitimate heir.

This sentence doesn't have a relative clause, but if I wanted to add one like "the heir that the lawyer discovered", how would I apply this lesson's inversion rule?

You would say l'erede che ha scoperto l'avvocato. By placing the subject (l'avvocato) after the verb (ha scoperto), you put the natural focus on who did the discovering. This sounds much more native than the direct English translation l'erede che l'avvocato ha scoperto.

Could I translate "during the reading" as durante leggendo?

No, in Italian you cannot use a gerund (like leggendo) directly after the preposition durante. You must use a noun, which is why the sentence uses la lettura. If you wanted to use a verb instead, you would change the structure to use mentre (while), as in mentre leggevamo (while we were reading).

The word erede ends in an "e". How can we tell if this specific heir is male or female?

We know this is a male heir because of the masculine article un (with no apostrophe) and the masculine adjective ending -o in legittimo. The noun erede is the same for both genders. If the heir were female, the sentence would use an apostrophe and a feminine adjective: abbiamo scoperto un'erede legittima.

Is scoperto an irregular past participle?