I'm not sure if I missed this somewhere in the rules, but if a non-unique open group gets defeated and the card is returned to your hand can it be deployed again during the same mission if you spend the Threat? For instance if I have an Imp Officer and he gets defeated and I've got the 2 Threat to deploy him next time I get the chance, can I?

Yes. Open Groups is in game terms quite equal to the cards in your hand of deployment cards.

Campaign Deployment, p.11 said:

When an initial or reserved group is defeated, except for unique figures, the Deployment card is returned to the Imperial player's hand and can be optionally deployed during the Status Phase.

Well, during any optional deployment, not just Status Phase.

Also note that you can reinforce during an optional deployment (second page of the campaign book).

imp stands for imperial does it not? Why did you use a descent card with a reference to imperial assault?