When is 'get' interchangeable with 'be' and do we use 'have' only when it has a causative meaning?

There isn't any rule for get being interchangeable with be. Get is already in a paradigm with be.
As well as with have.

Get is an auxiliary verb, and it functions as the inchoative/causative form of both the auxiliary verb be and the auxiliary verb have.

• Bill has the flu. ~ Bill got the flu. (inchoative of have)
• Bill is sick. ~ Bill got sick. (inchoative of be)
• Bill has a car. ~ Bill got a car. (causative of have)

So it's not interchangeable with either be or have, but when dealing with persistent predicates changes in the past affect the present and future. If Bill got a car last year, chances are he still has it. If he got sick last week, he may still be sick.

So they look like they're interchangeable, but that's not the way things work. Nothing is interchangeable. Sentence grammar is not a matter of adding words like beads on a string, changing them at will. But it's not a box of beads; it's more like a junk drawer of mismatched screws, bolts, and hinges that will miraculously fit together at speed (if you use the right tools to assemble them and don't put them under too much stress).

There are also at least adversative and causative have constructions, with or without Raising.