A lot of summonable spirit powers are actually Magic + Attribute tests such as Fear, Confusion, and Influence. Others have their effect dependent on magic such as Concealment, Movement, Immunity, and Elemental Attack. Now in the vast majority of cases, a spirit's force is the same as magic. If there was a way to increase magic directly, a spirit could have more magic without an increase in force.
As for adepts while possessed, the traditional answer is "yes adepts get to use their powers while channeling/possessed." That's why "mystic adept channeling face puncher" has been a fairly common and complained about build. However, I'm not sure if that's the case since adept powers run off PP not magic directly the vast majority of the time.