Starcraft – install Starcraft 2 multiplayer only, without campaign

It sounds to me like you've missed the secret mission "Piercing the Shroud" and the alternative choices on the Tosh/Nova Selendis/Hanson and Warfield/Tychus missions. For the latter three at least, you should be able to complete the missions by going to the mission archive and selecting them from the menu.

Alternatively -- did you do all the available missions before going to Char? If you didn't you'll need to start a new campaign to finish those -- once you go to Char, you can't go back and do any missions you may have missed.

It might be worth it to check your "Wings of Liberty " Achievement progress and list exactly which 4 missions you have yet to complete (The achievements that award Warfield, Mengsk, and uninfested Kerrigan portraits, for reference).

Edit: Okay, it looks like you missed the Zeratul Missions. After completing "The Dig" there is a new item in the Lab screen, in the lower left-hand corner that has "Crystal" or some such identifying it. Clicking it will let you take 4 Zeratul missions. It looks like you only did the first.

Reaper are neither a meaty nor a support unit, meaning they are not good in an army composition. This is because of their cost and their build time. Reapers are good scouts and harassment units.

There are a few timing attacks that are nasty, but actually mass Reaper is not viable. So Reaper are reduced to:

• Scouting: especially with speed they are a cost effective scout compared to scans or slow units like Marine / SCV that get killed all the time.
• Harassing: Having a few Reaper that regularly visit the opponents mineral line can be compared to drops. The opponent has to stay in his base or he risks loosing his economy.

But you also need a real army, that can finish you opponent.

In HotS the Reaper has exactly the same role. Their building damage is reduced, but they get an out of combat healing ability, so they can harass more often without dying.