Entity orientation is always in respect to Earth fixed (ECF), if you don’t specify an orientation, it default to ENU at the entity position (so that volumes and shapes have a decent default orientation).
The lack of a modelMatrix is intentional for a number of reasons (ease of use and time-dynamic position/orientation are the two that immediately come to mind). It’s true that losing the model matrix means you can’t apply a certain transformations like you can with a primitive, our goal is to have parity between functionality when it’s needed. So if you have a good use case for something that the Entity API can’t do, just let us know.