Learn German: Wir haben beschlossen, früher zu fahren.

This sentence is a reduced version of Wir haben beschlossen, dass wir früher fahren. Because the subject (wir) is the exact same in both the main clause and the subordinate clause, German allows us to drop the second wir and use a zu infinitive clause instead. It sounds much more natural this way!

Could I use this exact same structure to say 'We decided that he should leave earlier'?

No, you can't. The main rule for reducing a dass-clause to a zu infinitive is that the implied subject of the infinitive must match someone in the main clause (usually the subject). If the people are different, you must use a full clause: Wir haben beschlossen, dass er früher fährt.

Why is fahren translated as 'to leave' here instead of 'to drive'?

While fahren literally means 'to drive' or 'to ride', German speakers frequently use it to mean 'to leave' or 'to head out' when a vehicle is implied. If the group were walking, they would likely say früher zu gehen instead.

Is the comma before früher strictly necessary?

Yes, it is standard here. While commas before bare infinitives (like just zu fahren) are sometimes optional, the comma becomes mandatory when the infinitive phrase is expanded with other words—in this case, the adverb früher.