Carlo - you missed that individual planetary navies are expected, and not answerable directly to the IN, but definitely supervised by the IN. "... any world with a tech code of Early Stellar (Tech Level 9+) and Moderate Population (Population 4+)." (Reb SB, p 29). ...
Reb SB also says planetary navies are not jump capable, ergo confined to activities within their home systems. It'd be interesting to discuss their place in the scheme of things, but I didn't consider it relevant to the immediate issue as their reach is clearly limited to their own systems.
... Keep in mind, tho: CT sources are based upon the 3I in 1100 to 1108.
MT is 1117 to 1135
MGT is 1105-1107 so far.
There are no "Duchy Navies" in MT ...
Ah, I see that. MT character generation identifies the Imperial Navy, Reserve Navy, and System Squadron, whereas CT High Guard II had the Imperial Navy, Subsector Navy, and Planetary Navy. One infers that sometime between the end of the Fifth Frontier War and the supposed assassination of Strephon in 1116, there was a major reorganization of the Colonial/Subsector forces.
As of CT, the subsector was the basic unit of Imperial government above the planetary:
Supplement 3, pg 2, "subsector: An artificial mapping concept intended to delineate and define locations within the galaxy. In most cases, the subsector is used as a governmental unit within the Imperium. ..."
Subsectors were governed by subsector dukes:
Supplement 8, pg. 7, "Interstellar government begins at the subsector level - on one world designated the subsector capital. The ruling figure at the subsector capital is a high-ranking noble selected by higher levels of government. This duke has a free hand in government, and is subject only to broad guidelines from his superiors. But at the same time, the duke owes fealty to the higher levels of government, ultimately to the Emperor himself."
With the presence of a subsector unit and a subsector government, it was logical to conclude that the subsector duke held authority over the subsector fleet except in wartime, as portrayed in
Fifth Frontier War.
The introduction of MegaTraveller brings a change, but that change cannot be backdated too far before 1117 because that is when MT starts its milieu. Ergo, CT's description of the milieu remains canon until some unstated date before the assassination, and most likely some date after the Fifth Frontier War ends (since even if it had been started elsewhere earlier, it could not be implemented in the Marches until the war ended).
... the subector duke's authority is limited basically to oversight of the other nobles...
Yes, as of some undefined date between the end of the Fifth Frontier war in 1110 and the supposed assassination of Strephon in 1116. We cannot state with certainty that it was limited before that.
... Hans' insistence that the Duchies are foundational is not supported by and is counter to MT sources. ...
Which address the state of affairs in the Marches after 1116, or rather after some undefined date between 1110 and 1116. Whether or not it is supported by pre-MT sources depends on what you read into the data I presented above.
There is also some change in the sector government organization, but I'm not familiar with the sources. All I know is CT talks about the subsector as a unit of government, and then the Wiki starts talking about duchies that in some cases appear to encompass more than one subsector and fold in subsidiary counties and baronies. I'm not familiar with that structure, what it evolves from, or how it relates to the CT structure.