The subsector navy is run by the subsector gov which is a sub-division of the Imperial gov system of Domains, Sectors, & Subsectors.
Thus NO amending is necessary.
It could be, but that is not what the early version of HG said. Or at least if it was trying to, the way they worded it is not conducive to that being the only valid interpretation.
The subsector navy/colonial naval units are well-documented thru-out many CT & MT piubs & also present in the 5FW boardgame.
I'd agree with that, but we've all recognized that at different times, different things were clear and other things were not. And sometimes the later things are really likely to be clarifications or expansions.
I wouldn't say that the early HG print meant anything other than what you suggest, but it certainly didn't word it well if that was what they were aiming for (which I think likely).
Also, the subsector govs possess taxing authority as documented by the method used to fund one of the Azhanti High Lightening cruisers (Children of the March) when it was originally built during the ThirdFW/SolRimWar era.
But that just muddies the waters further to my mind if it was stated as such. The subsector should not *be* an entity that can tax, except as a direct hand of the Imperium.
The subsector fleet is NOT actually separate from the IN any more than the US NationalGuard is actually separate from the US Army or USAF.
That's a latter day expansion of US powers that is still debated by many. There were "<state name> NG" but they were formally under the control of their state Governors who could determine if they would go somewhere or not. In the 1980s or maybe early 1990s, things started to change.
In fact, the subsector fleet is subordinate to the IN.
Subordinate is not necessarily the same as part of. Subordinate can mean you have been attached to a higher command that someone else's forces may be the higher command authority, but it doesn't necessarily require you to be a part of.
So, it is NOT at all clear that the subsector fleets are 'different' beasts.
I'd agree that several aspects are unclear. Your position is not unlikely to be correct and it may conform with later clarifications.
Sometimes the ambiguities of English as a language are something not paid enough attention to (author knows what he wants to communication, so sees what he writes through the already informed lens and thus misses ambiguous language). I have French friends who lament that ambiguity in English; Many times they have been annoyed that they said something in English, it had two or more possible interpretations due to our grammar's permissiveness - noting that in French, there would have only been one way to say the thing and it would have not been ambiguous.
A fact that the 5FW boardgame makes abundantly clear.
It's hard sometimes also to be certain whether something is a change or a clarification. We have seen both in the body of Traveller writing. Even the original authors suffer the human reality of memory sometimes being fuzzy and sometimes saying contradictory things.
Again, don't necessarily disagree with your interpretation.
As documented in the 5FW boardgame, the subsector fleets are composed of older (TL14) IN fleet assets.
That also would not necessarily of itself mean they remained Imperial assets. Look how US police forces now have gear used in Iraq and Afghanistan.
Which make them comparable to the Zho Navy (TL14) & superior to the Vargr/SwordWorlders (TL13) fleets.
I'm sure that the 'local' system navies vary quite a bit.
Could do, if it is truly a patchwork Imperium with very different tech levels contributing fleet elements. Then again, at some point, the gap is enough that you'd not want that in your mobile fleets, just in planetary garrison fleets.
5FW also follows the 'tripartite' system with ground units.
There are Imp regular & colonials of varying sizes ranging from Batt's to Armies (regulars are uniformly TL15 while the colonials vary from TL14 down to single digits depending upon the system where they are stationed) while system defense forces (expressed as Batt equivs) are abstracted, static, & also carry the TL of the system where they are stationed.
I would imagine part of that mirrors 19th and early 20th century colonial military organization.
As far as forces w/i the Imperium goes, IMO, one of silliest things about 'The Rebellion' was how a backwater area like Illelish, with no real frontier, suddenly generated massive military assets practically overnight. Assets that hadn't been needed for centuries & centuries.
Then again, if we sat and looked at the number of high pop, high tech worlds in the domain, I bet you could crank out a lot of vessels in short order if you had the budget.
We sort of assume the Imperium was monolithic and stable.
If there are regular insurgencies, rebellions, revolts and seperatist regions, that would justify more internal force levels.
If there were rivalries and concerns of actions within the Imperium between Imperial worlds or sectors or whatever, that too might militate for stronger internal force levels.
That said, I do think the interior except for Capital area itself would likely have lighter military forces - except in worries of Civil War/Revolution on a large scale, most main line units would be needed on the outer perimeter (also the expansion areas).