Learn Russian: Неужели этот слух об их разводе — правда? - Is this rumor about their divorce really true?

Неужели adds a strong sense of shock or disbelief, translating roughly to "Is it really possible that...?" If you used разве, it would sound softer, like you are just double-checking a doubt. Here, the speaker is genuinely surprised by the divorce rumor.

Why is the preposition об used here instead of just о?

The preposition о (about) changes to об when the following word starts with a vowel. Because the pronoun их starts with the vowel и, we use об to make the pronunciation smoother.

Why is there a dash (—) right before правда?

The dash acts as the invisible present tense verb "to be" (is). In Russian, when both the subject of the sentence (слух, meaning rumor) and what you are calling it (правда, meaning truth) are nouns in the nominative case, you connect them with a dash.

Правда is a noun meaning "truth". Why is it translated as the adjective "true" here?

In English, we usually say a rumor is "true" (using an adjective). In Russian, it is very common to equate two nouns to express this, literally saying "this rumor is the truth". You could use an adjective like правдивый, but using the noun правда is much more natural in conversational Russian.