Are these going to be (and did you get for Namibia) full map sheet scans?

This might not seem overly relevant for OSM use but for the value of such data as a record of cartographic history the information on the map sheet rims (like data sources used, specification of map sheet edition etc.) is of immense value.

You have not even mentioned the more fundamental issue with comparison of different classification systems - that they always re-cast one classification into another and this way create an inherent bias. And when comparing OSM data it is always the OSM classification that gets re-cast into the other one - with the inevitable losses in semantic accuracy.

W.r.t. ground truth sampling as the gold standard for quality assessment - that is only the case if you actually use true unbiased random sampling. And i have yet to find a single such study that does this. Most studies do not even document the method of selecting sampling locations and hence qualify as pseudo-science (for global analysis for example: picking a uniformly distributed set of random sampling locations is non-trivial - not to mention that ground checking a truly random set of sampling locations is very expensive). By manipulating the sampling, even in a subtle way, you can essentially freely modify the results of such a study.

There are two persistent urban myths w.r.t. boundaries:

• Boundaries are universally defined in some abstract coordinate space and stay there irrespective of changes in the physical world.
• Boundary data sets published by public authorities invariably represent objective truth about boundaries.

Both of these are wrong.

Many (if not most) boundary data sets published by authorities are generalized approximations.

Most international boundaries world wide that follow a physical geography feature (either a river or a watershed divide) are legally (by agreement between the countries involved) tied to that element (Using formulations like: From A to B follows the course/centerline/left side/right side of the river X). Often a more explicit specification of the boundary is done (and demarcated) by a boundary commission, sometimes subject to regular revisions.

What counts in OSM is of course in any case only the line of de facto administration - which is usually well verifiable for international (admin_level 2) boundaries but increasingly non-verifiable for the higher admin levels.

In my experience most mappers treat higher level administrative boundaries as abstract artefacts and ignore them when editing other data and do not connect them to other geometries even if they are functionally tied to them.