Split-screen co-op is available for Arcade mode. It doesn't cover all of the maps (the biggest maps, designed for Galactic Assault, are not available), but there are several different scenarios (10+ each, light-side and dark-side) available, and some of them look like they would be a lot of fun for co-op. Vader and Palpatine, assaulting the rebel base on Yavin 4, would be one of those. You can also make custom scenarios as well, and adjustable difficulty. There are a number of achievements for Arcade mode, but they put a daily cap on how much you can earn from it.
I didn't think the campaign was all that short. I spent 2 long nights playing through it, totally glued to the couch, spending something more in the ballpark of 8 hours. Maybe it can be blown through in 4 hours, but that would be by kids with ADD, trying to race through it.
The truth is, the days of longer campaign experiences in a AAA game like this are in the past. The kids these games are primarily marketed to do not want them, and they take way too much time (and therefore budget) to make these days. The more powerful the hardware gets, allowing for better looking games with more detailed environments, the fewer levels we see, whether single or multi-player. This has been the trend for decades now, and I don't see that changing. These days, it would take an exceptional budget (in the ballpark of $200 million+) and 4 or 5 years to develop a modern AAA FPS game with the number of levels and long campaigns of years past. That's a big risky situation, when the biggest sellers they have to compete with get roughly half that time and budget.
The only reason that a game like this got made, and got the hefty budget that it did (to include a single-player campaign at all, not to mention the planned free DLC releases), was because of all the $$$ EA saw if they could monetize it the way they planned to. It was never going to be otherwise, in this day and age.