Hello Des:
The crux of the matter is this: Your Namibian validation of your Canadian licence is based entirely on the underlying Canadian licence being kept valid at all times in accordance with Canadian aviation regulations.
In other words, what the Namibians (or anyone else in this world who issues a validation of a foreign licence) are saying is "OK, we will accept your Canadian licence to enable you to operate Namibian aircraft, on the basis that the Canadian licence would enable you to operate the same aircraft in the same manner/conditions in Canada". The Namibian authority is not passing any judgement on your competency, instead they are relying on the judgement & oversight of the Canadian authority who issued your licence. It is therefore essential that you keep your underlying Canadian licence valid in accordance with Canadian aviation regulations.
I retired two years ago, hence I am not up to date with current CARs about keeping an instrument rating valid. During my time in the industry (1970s-2013), we had to do an instrument rating checkride with a person who was designated by Transport Canada as an examiner (or, with a TC employee who was an examiner) once a year to keep the IR valid. I think the rules have changed since then.
The important concept that you must appreciate here is that you have to fully comply with Canadian rules to keep your Canadian licence (upon which your Namibian validation is based) valid in every respect. This means your medicals have to be carried out by an AME who is accredited by Transport Canada, your IR renewal checkrides have to be carried out by an examiner who is accredited by Transport Canada, etc., exactly as if you lived and worked in Canada.
Here's an easy way to see if you are legal or not: Ask yourself "If this was a Canadian registered aircraft, and a TC inspector appeared on the ramp this morning and examined my licence documents with a fine-tooth comb, would I be legally qualified to operate it?"
You are not employed by a company who is a "Subpart 4 of Part VI private operators or Part VII air operator" (as you wrote in your first post), simply because you are not employed by a Canadian company. The Namibian operator you fly for is not governed by those Canadian regulations. Transport Canada exercises no oversight of your Namibian employer.
The foreign instrument proficiency check that Ramjet referred to in post #2 above implies a checkride that keeps a foreign licence valid. If you had a full Namibian licence - in other words, a Namibian licence that stood on its own two feet, not a validation that is based on the underlying Canadian licence - then you have an IR checkride carried out in accordance with Namibian regulations to renew that Namibian instrument rating, and you subsequently showed your Namibian licence with the record of that IR checkride to TC, TC would then accept that as sufficient evidence that you have kept your Canadian IR valid (assuming, of course, that TC has a reciprocal recognition agreement with Namibia).
In my case, I had a full Swiss (later JAA, later EASA) licence that stood on its own two feet, I kept it continuously valid for many years, and when I later returned to Canada, I showed that evidence to TC and they accepted it for the purpose of declaring my Canadian IR to be 'current'.
Hope this helps you understand things.