前もって and 早めに: advance preparation and acting early
Compare notifying someone before a visit with leaving early, separating preparation for a later need from an earlier time.
Preparing before something is needed
前もって, read まえもって, identifies action in advance. 訪問前に、前もって連絡しました。 says that contact was made beforehand, before the visit.
前もって資料を読んでおいてください。 asks for the material to be read in preparation. The focus is readiness for a later situation.
Acting somewhat earlier than a reference
早めに, read はやめに, means somewhat early relative to a reference. 今日は早めに出発しました。 says that the person left early today.
早めに寝ましょう。 suggests going to bed early. It does not explicitly identify a later activity being prepared for.
Overlapping contexts with different emphasis
Both 席を前もって予約しました。 and 席を早めに予約しました。 say that a reservation was made. The former highlights preparation in advance; the latter highlights allowing time by booking early.
前もって知らせてください。 asks for advance notice without stating a fixed number of days. Check separately how much notice is needed.
Does 今日は早めに出発しました。 establish that the materials were also read beforehand?
Show the answer and why
No. It says that the person left early today. It supplies no information about preparing the materials.