First off I have to say this was an awesome thread with various ideas of how the 3I's military would work against another force. The discussion of meson cannons and the communication networks were excellent, and definately applicable to a planned attack on a world by the full might of the Navy.
However, it does seem that the delay of Travel, communications and the simple act of refueling seems to be somewhat ignored. I'd love to hear your thoughts of the following:
Frontier space, say even the Spinward Marches. Even with a significant presence in the Regina subsector and around Jewel, it didn't stop the last Frontier war from spilling over into 3I space. It even mentions in the Spinward Marches books that most the subsector patrol fleets are old. They aren't these TL 15 meson equipped units, but they are probably going to be the first to respond to any problems.
So ground troops, Grav-Tanks, all those might be used not because they are optimal, but because they are 1.) Available, and 2.) Cheaper. War costs money, but keeping your massive thousand solar system spanning Navy in the best ships, with the best equipment.. is impossible. Just look at the age of the majority of our RL war technology. True there are a lot of really nifty weapons out there, but it still comes down to M16s and tanks.
On the idea of control of space, that is assuming you have the stronger force in the area. I could easily see the following scenario being common if a war was just breaking out.
40 year old battleship "Stormgard" comes out of J-Space on it's usual patrol through a system. The battleship runs with a few small, older support vehicles and enough fighters to deal with the pirates they might come across. Sensors pick up similar sized group of ships refueling off the gas giant, who don't respond to hails. Soon the two forces are engaging.
The Stormgard realises it's outgunned when another battleship arrives from j-space, a little slower than the other ships. It is approaching and will join the engagement in a few hours. The Stormgard wont be able to outrun the other ships, but it keeps them off long enough to launch a scout to jump and warn the closest naval base, four parsecs away.
Even if that scout hits another patrol on it's jump, reinforcements are still weeks away. The Stormgard launches lifeboats to the systems world, a small frontier colony.
Reinforcements arrive to find the battleship long destroyed, and two enemy battleships, one trying to destroy any small craft that may attempt to escape the planet, the other near the gas giant, a source of refueling for the enemy fleet that will be jumping in soon.
This time the Imperium's force is larger, but they still don't have their full might. After all, it was just a frontier base. The enemy has launched ground troops to try and control the ground of the colony, the resources would be able to reinfoce the coming fleet.
Space combat resumes, the ground forces cut off as the battleship moves join with the other enemy ship, regrouping before they clash.
The rest of the initial enemy invasion jumped further in, moving at a slow jump-1, but soon reaches that naval base which is now without much defense, destroying logistic support.
----------------
Could continue on and on, with each group pushing the other back and forth. Ground targets being useful to control or destroy, and neither force has the best tech. Once word reaches those top end fleets, months have passed. The winners of that battle will already have been decided.
So, it's great to discuss the depth and awesomeness of ideal situations in war, but plans never go as planned. I know there is a quote about plans and battlefields somewhere..

Most wars are fought with whats available, and when reinforcements are weeks to years away, you use what you have to get the job done.