Open question 1: Is it useful to differentiate non-modular units that
import other modules (header or named) from the ones that don't? If
so, what terms do we want to use?

From a tooling perspective - possibly. The former ones establish an interface dependency whereas the latter ones don't. May be BMI-dependent vs. independent?

Open question 2: What do we call a "Module partition" which "does not
contribute to the external interface"? We seem to have a contention
between "Module Internal Partition Unit" and "Module Implementation
Partition Unit".

"Internal" seems to the point, "implementation" covers only a subset of its use cases.

Ciao
  Dani