The type S is cheap, not so much with the J-6 fleet courier, but 2 parsecs or 6, there still is a week travel time. If they hit a fortified world on the border, you really arn't going to need so much forewarning, just form fleets and attack. I still think you will have your fleets staged back, and no matter what, the initial phases of the war will happen in a 2 week to month window of forming and attacking, it is only a question of how deep the enemy car get in that time. Though with the build time on capital ships, I think the fleet will be held back in protected zones, worlds can hold out for years, but if your fleet is destroyed, it's a long time building another one, even though I haven't seen a huge amount of info on production capacity.
True about the Scout vs courrier matter. It's because of that I said courriers would only be used in high thread areas.
This week you may gain in J6, though, could allow you to ready any combat ship that could be in maintenance (about 5% of your SDB fleet at any moment, if you rotate them for annual maintenance) and to position all your ships in strategic places, withrawing them from customs duty to put them on war duty, as an example.
Noneless, I guess you're right about most of the battle fleet being deep on Imperium (Corridor Depot, Trin, Rhylanor, etc) to be sent for latter stages of war, as you say.
About worlds holding for years, it depends mostly on the habitability of the world and the determination to take it. Earth holds for nearly 9 months to a determined assauldt by the IN. Glisten could have to surrund quite early if really thertened, as any bombing would be catastrophic.
About production capabilities, neither I have found much information (though it can be extrapolated from TCS, and if so it's enormous), but about building times there is some information, and you're right, if you lose your fleet, you may well not be able to have more until the war is over, unless you have ships to transfer from other places.
As the same applies to both sides, I guess if war is not reolved quite quickly it will resort to a protracted attrition war, as both sides will try to avoid losses.
It's also worth to say that, if combat is resolved as HG/MT tells, to keep the 'battlefield' after a space battle would cut your losses quite a lot, as in those systems quite more ships are dissabled (mission killed, but in reaparable state) than truly destoyed (not worth reapir), so the hulks you recover can be repaired, while if you retreat your hulks are gone for good (captured/scuttled or fully destroyed). I've comented that many times in other threads.