Learn Spanish (Latin America): Llegamos al mirador, desde el cual el paisaje era increíble. - We arrived at the viewpoint, from which the landscape was incredible.

You could actually use either! Both desde el cual and desde el que mean "from which" and refer back to el mirador (a masculine singular noun). However, el cual tends to sound a bit more formal or literary, which fits nicely when describing a beautiful landscape.

Since we are talking about a physical location, could we just say desde donde?

Yes, absolutely. Because el mirador is a place, saying desde donde ("from where") is very common and sounds completely natural. It is often used interchangeably with desde el cual or desde el que in everyday speech.

Why is the landscape described with era (imperfect) instead of fue (preterite) if the first verb, llegamos, is in the preterite?

This sentence perfectly shows how the two past tenses share the load. Llegamos (we arrived) is a completed action that moves the story forward, so it uses the preterite. The landscape being incredible, however, is not an action that started and ended—it is a background description of the setting. Descriptions in the past almost always use the imperfect (era).

Does cual ever need an accent mark in this type of structure?