I'm not sure this goes here, so apologies in advance...

I'm new to the game and just ran my first two missions this past weekend as the Imperials. I was able to navigate the rules manuals well enough to ensure we had a good time, but I'm confused on a few things including villain and hero packs.

Am I correct in understanding that if you buy these packs---and I'm not interested in anything but campaign right now---that Han, Fett, or any of the others are only useful in replacing the flat tokens or running a single side mission?

If so, what use are the characters after that? Can you freely insert them into other missions as deployments?

Since you build a side mission deck, isn't there a somewhat slim chance of Han or Fett even coming out?

What does Fett's mission text mean that the reward is Boba Fett himself? Does he immediately attack them where they are? Or they hire him? Sorry if I'm a bit dense on these things but I have a 3 month-old and just don't have the time to scour the boards and rule books like I did before.

I'm sure I'm wrong on this, but the packs seem like expensive luxuries if they only get played once or twice or might never hit the table if players don't choose them. My group saw Luke come up as a mission and immediately assumed it would be too hard and picked a character's personal mission instead.

The characters such as Han and Fett can be earned as either allies for the rebels or villains for the imperials. This means that after you earn them as a reward by playing their side mission, if you are Imperials, you can choose to bring Bobba Fett into a mission by paying his deployment cost in threat. For the rebels, they can start a mission with an ally on their team (Han or Luke) by letting the Imperial player start with threat equal to the deployment cost of the card. So rebels bring in Luke, Imperial player starts the mission with 10 threat. Also, rebels can only bring 1 ally and Imperials 1 villain in a mission.

You are right that there is a slim chance the side missions from the packs will come up. I don't remember the number exactly, but I think its like a 30% chance you'll see any certain side mission based on how many you play in a campaign and how big the deck is. We would just house-rules it and allow the rebels to choose one of their allies to attempt to get if they wanted to, since it's possible it could never even show up.

The imperial player can include more than one earned villain into their open groups, but it isn't usually wise, because it must be done before you know whether the rebels are taking an ally. If you take too high-cost open groups, you may not be able to deploy them during the mission, and are thus severely handicapped in your deployment choices.

Also note that if a villain like Bobba Fett is defeated, he cannot be redeployed in the same mission, though he could in following missions.

Correct, only non-unique deployment cards return to your hand when their last figure has been defeated. Unique figures can not be redeployed in the same mission by any means. (Even As You Wish allows you to only get the deployment card into your hand, not allow you to redeploy the figure in question.)