I tried cleaning the escapement again as the key sticks partway up the stroke at low tension  and I hit it pretty hard with lacquer thinner and later naptha also with a lot of brushing and canned air. No change at all. Since the q key works perfectly and very free when doing caps, when the basket is down, I still think that there is some hang up elsewhere. Haven't given up yet but I'm getting close.

Hopefully the OP will read this. Set the machine vertically on its back panel, then with one hand trigger the universal bar. Then gently press and slowly release the Q key. The type bar should move under its own spring tension, against the force of gravity, back to the rest position. If not, lubricate each joint in the linkage system, one at a time, with alcohol as you redo this test each time. You should be able to locate the offending linkage. Degrease it with solvent and a mascara brush, then repeat this test once dry to ensure it’s fixed.

For this problem only happening in lower case on the lighter touch setting, the linkage rod could be rubbing against a neighboring rod (like the A rod), since the angle of the rods changes slightly between lower and upper case with a segment shift design. If so, slightly file the interfering surfaces.

I’ve also seen this problem fixed by slightly bending the type bar at the pivot point near the curved fulcrum rod. Or rather, unbending the type bar, it may already be bent enough to not pivot freely.