Ensure release will include a TDS zip
Test prefix-to-number conversion with (non-integer) powers
Use expandable definitions for unit macros

This is what my comment actually says, and it's quite true: you need
expandable definitions for everything to work. There is a
\protected@edef to cover the case where LaTeX2e commands are involved,
so all should be well. (Certainly there is no obvious alternative route
that doesn't require a rather tedious mapping.)
Stick to "unit" for everything

This is a tricky one, but it's either "unit" or "units" all round!
Test for "unit/font-command" and direct setting

As direct use of the var is allowed for performance reasons, the two
tests should be identical here.
Test for "unit/inter-unit-product"
Test for "unit/parse"

Question: Is the output right here? This is the same behaviour as v2,
but I wonder if there is a way to get things right without parsing.
(Almost certainly not: the entire point here is that to understand a
unit you have to parse it!)