It seems implied that the infrastructures of the three players (UEF, GTVA Terrans, GTVA Vasudans) are all fairly similar. The UEF is able to hold the line (mostly...) against Steele's force. Most of the reason I assume the war goes badly for the UEF is because 1) the GTVA was preparing for the attack and the UEF wasn't, 2) Steele is crazy smart, and 3) the Elders and Byrne want defensive action.
The type of structure would depend on the habitability of the planets. If you have "shirtsleeve" environments, then you don't need stations/habitats/random rocks on which to live. In that case, most traffic would be on-planet, and the planets would be more densely populated. If you have Mars- or Venus-type planets that require domes and/or terraforming, you're essentially living in a station already. So, I'd imagine there would be more stations floating about and therefore more space shipping. I would agree that it would be denser near the homeworlds, as those were probably developed first.
If Sol housed half of the human population, then the colonies are fairly sparsely built up. I'm thinking less than 2-3 billion people on-planet per system. I don't recall any population numbers from FS2 or BP, so I could be way off. There would be plenty of open space and raw resources on-planet given this, assuming the average size of each planet resembles Earth. You would assume that each system wouldn't have a huge manufacturing base compared to Sol; it'd be spread among the systems. Sol had I think at least 3 shipyards in-system, so you'd figure maybe 3-4 main shipyards outside of Sol.
Since the end of the Second Incursion, the Terran half of the GTVA has been solely focused on getting back to Sol. It's mentioned that they've neglected other areas to achieve this, so there may be decaying infrastructure around the far colonies. 50 years is a long time to be without the umbilical to home, and 18 of those focused on one project.
Tactically, I don't think there would be much difference if the roles were reversed. If the UEF committed to offensive action, they'd still be using hit-and-runs and stand-off weapons. They can't match the Tevs in a knife fight, so they'd try to snipe from distance. Calder's strategy would probably to hit them hard in weak positions, with locally overwhelming force. If the UEF could bring the war outside of Sol, they might be able to achieve parity in tech as the pressure on their own infrastructure would ease. In that case, you might see more "conventional" FS warfare.