Learn Spanish (Latin America): Estamos por sobrevolar el valle.

Both are used to express that an action is about to happen very soon. Estamos a punto de translates perfectly to 'we are on the point of' or 'we are just about to,' making it feel a bit more immediate. Estamos por simply means 'we are about to.' In everyday Latin American Spanish, they are practically interchangeable for imminent events like this one.

Can I just say volar sobre el valle instead of using the compound verb sobrevolar?

Yes, volar sobre and sobrevolar mean the exact same thing here. Using the compound verb sobrevolar sounds a bit more precise and is very common in aviation contexts (like a pilot making an announcement), but volar sobre is perfectly natural too.

Since volar has an o-to-ue stem change, does sobrevolar also change?

Yes! Compound verbs keep the same irregularities as their root verbs. If you needed to conjugate it in the present tense, it would be sobrevuelo, sobrevuelas, and so on. However, because it follows the preposition por, it must stay in the infinitive here, so you don't need to apply the stem change in this specific sentence.

I've seen estar por used to mean 'being in favor of' something. How do I know which meaning applies here?