1. 20 planets might not seem like much to begin with but think about it. There will be two battles to control each planet, one space and one ground battle and if each battle takes 30 minutes to an hour, you're looking at a good 20-40 hours of gameplay time and that doesn't even include the time spent between missions on the galactic map. I will admit that I hope they include an editor that allows custom planets to be added.
2. The game setting means that the Republic and Seperatist forces no longer exist as such. I suspect that left over forces from both sides will be included in the Imperial and Rebel unit selection. I do think they could have had a third smuggler side though.
3. The TIE Crawler is most likely a cheap unit. Think of it as a light tank from the original Command and Conquer. Also if you look at some of the screenshots, you will see speeder bikes.
4. They said that the space and ground aspects of the game will affect each other, not take place at the same time. Example: if you have an ion cannon on the planet, you can fire it at enemy ships in a space battle. This will most likely function like the super weapons from the various c&c games.
5. I keep saying it. The game only STARTS before ANH. Time will pass as the game continues. Not only that but giving the player access to all technology at the start would be boring. It would make more sense for players to either research new units, or be given access to new units as time goes by.
Missions such as stealing the X-Wing prototype would mean the player gains access to the X-Wing earlier than they would by researching/waiting for it.