[geraldkw]

Can we get a glossary entry that explains whether you can or can't solve a mission that has already been scouted and had a objective completed there, and possibly another one that explains whether you can complete a objective targeting a mission card where the mission has already been solved?

I think there has been an official ruling at some point but I can never remember it and I can't find it anywhere in the glossary.

[AllenGould]

That's because they're orthogonal, except for this:

Mission attempts may not be made at assimilated planets, and the mission affiliation icons become irrelevant (facilities requiring a matching affiliation icon may no longer be built there).

Beyond assimilation locking out mission attempts, missions don't care about objectives or vice versa.

[9of24]

Additionally, from the Borg entry (specifically 3rd paragraph of Objective heading):

When you play (or activate) a Objective card, you must immediately target an appropriate location, ship, personnel, etc., as specified by the objective. Objectives may target solved or unsolved mission locations. The objective then allows your Borg to scout the ship or location, initiate battle, abduct a target, etc.

[geraldkw]

Moral of the story, don't use stealable missions or if playing borg, don't solve them until you can win by solving them.