Can a model have multiple focus tokens assigned?

No.

"Conditions", RRG, Page 9:

A figure cannot be affected by multiple instances of the same condition. For example, if a figure that is already Stunned would become Stunned again for any reason, nothing happens.

Can you use "Show of Force" in between of two actions?

Just want to know if the E-Web in 1st Mission would be able to shoot 2 rainbows in the first round after deployment.

Yes.

The trigger on Show of Force is "when an Imperial Figure declares an attack". Since the E-Web can attack twice, it would spend its "free" focus from the event trigger on the first attack. Once the first attack is resolved, then you would be able to use Show of Force to give it Focused again.

When you do this, after saying "TASTE THE RAINBOW!!!" on the first attack, you have to then follow up with "A DOUBLE RAINBOW!!!!! WHAT DOES IT MEAN????" on the second attack. Your Rebel players will love you.

I think my rebel 'friends' will quit game after mission 1 ...

While going over the rules and stuff I doubt they are going to win any mission.

A rainbow from an elite E-Web is mean!