These are all fine on their face, but I don't think any of them address the real issues with fleet combat in Traveller, and particularly the Rebellion.
See that most of what you say here would be very difficult to adress in a game, and even FFW has issues on them...
The first is command and control. Simply, with command limited to speed of travel, far ranging and dispersed fleet actions are extraordinarily difficult to coordinate. Fleets become more raiders and skirmishers, limping back when exhausted. Just getting disparate units to mass to a particular point takes a lot of time. This works fine against fixed targets, but it can certainly slow down the tempo of the campaign when it takes weeks for orders to travel and months to gather the forces together.
This will be scale dependent. On a game on a subsector scale, this will be a litle less than in one with system scale, as fleets are assumed bo have detachemnts across the subsector (and probably raiders/scouts in the adjacent ones) and low level skirmishing will be constant.
Similarly, supply is difficult to manage. Again, the fleets will basically be wild raiders, "living off the land" as best they can, arriving in system with their supply chains, hoping to scavenge supplies. Since it's the imperium turning on itself, seizing ammunition depots are a primary objective early on.
I agree the most critical supply (at least if combat is based on HG/MT) would be ammunition, and that means nuclear missiles. And it's likely the begin to be in short supply quite soon, as they are consumed in very large numbers in battle, and I guess they need also time to be produced (and, as you say, with a mobile front supply will be difficult). This will make beams very important quite soon, as they don't run out of ammo.
But this is for another game scale...
Engaging the enemy can be spectacularly difficult. The only way to engage them is to make them defend something. It's very simple for a fleet to flee, and dive deeper behind the lines. The individual systems need to have their own security to deter a fleet, or at least delay them, as chasing a fleet around is quite complicated. When a fleet jumps out, the pursuer has no real way to know where they've gone to follow them. In theory it's limited to astrography, you can look at a map and guess to which star system they've jumped to. But dense sectors provide a wide variety of destinations, making the hunt very challenging. Fleets with carried fuel, the fleet can skip systems and bounce through empty space.
Again this is less a problem in a subsector level scale, as it is assumed not to be a system's battle, but several engagements along the subsector.
Attrition is the next problem. Combat is very lethal, and ships die quickly. Ships are extraordinarily expensive, not just in MCr but, more importantly, in time. Large ships take forever to build. One problem with the Imperium is that the fleets are very old, and very established. There's never been much evidence of how much turnover the fleet has with retiring old ships and adding new ones. So, it's not clear how many ships are "in the pipe" when the rebellion started. But anecdotally, it seemed the fleet did not have a large turnover, with line ships approaching 50+ years old. There may well be a large yard capacity, but its less clear how much of that capacity was being utilized to build line ships at the start of the war.
In the time that new construction can start floating out of the ship yards, several years, "historically", the powers have exhausted and burned through their fleets. Imagine starting a game where you first reinforcements start coming on turn 150, or even turn 40 if you're doing month turns, and you can visualize the mad dash to consolidate borders and how quickly the powers can stalemate. The problem wasn't so much the defenders, as it was that the powers couldn't afford the losses from offensive operations. Each power has an almost fixed set of resources that gets consumed very quickly
Here is where i mostly disagree with you. If you base the game in HG/MT, mission killing is quite easy, but outright destroying a ship beyond repairs is not. Unlike wet navy, a crippled ship does not sink to the bottom of the sea to be irrecoverable, but will float in sapce to be claimed by the winer and, in most cases, quite easily repaired and reffited for winner's use (and more so as all sides use similar designs).
Sure, little new constructed ships will come until quite a time latter, but demothballed and repairs will make recieving more ships not such a uncommon event. My guess is that quite soon the problem will be more in trained crews than in ships themselves.
As said, all of this is by thinking on HG/MT combat paradigm...
Finally, we have almost no information about fixed planetary defenses. Beyond the concept of the "deep meson site", we don't have any information on how those are deployed, how many of them are deployed, doctrine in their use, how they are approached and destroyed. What of planets without DMS, are they limited to missile swarms (since most of the energy weapons are badly degraded by the atmosphere). Is a squadron or two of tiny SDBs the de facto system defense artifact?
At subsector scale, I feel it irrlevant. Subsector defense are a mix of subsector fleet, planetary fleets and fixed defenses, as occupying a subsector is a mix of space and gorund opperations across all of it.
