Yes, Dia akan datang besok pagi is correct and common.
You can even combine them as Dia pasti akan datang besok pagi, but everyday speech often just uses one of pasti or akan.
Dia pasti datang besok pagi is the most straightforward order. However, pasti is quite flexible:
All are possible and natural; the differences are mostly about emphasis and rhythm. Putting besok pagi at the front (Besok pagi dia pasti datang) slightly emphasizes the time (As for tomorrow morning, he/she is definitely coming), but the basic meaning stays the same.
Both can mean tomorrow morning, but besok pagi is far more common and neutral.
In most situations, learners should prefer besok pagi.
Yes.
So dropping pagi simply makes the time less specific while keeping the same structure and meaning of certainty.
Yes. For someone respected (an older person, a teacher, a superior), you can use beliau instead of dia:
Another written, somewhat formal form is ia, but ia is less common in everyday speech and doesn’t carry the same respectful nuance as beliau. Dia is neutral and fine in most casual and semi‑formal contexts.
Dia is singular: he or she. For they, Indonesian uses mereka.
Context may sometimes be ambiguous if you don’t know how many people are being discussed, but grammatically dia is one person.