JointTrajectoryPoint time_from_start Question

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Does it mean that the JointTrajectoryPoint should already be done executing when the time_from_start value is reached? Or does it mean that the JointTrajectoryPoint should start executing once the time_from_start value is reached?

A JointTrajectoryPoint encodes the state of the system at the delta-t from the start of the execution of the JointTrajectory.

Personally, I don't believe it's possible to "execute a JointTrajectoryPoint", as it's a point, but if you're asking whether the manipulator should have traversed the trajectory segment defined by pt_(n) - pt_(n-1) at pt_(n).time_from_start, then: yes.

In your words:

the JointTrajectoryPoint should already be done executing when the time_from_start value is reached

Edit: it might help if you see JointTrajectory not as a series of motion commands (which they aren't), but as a data structure describing a spline through the JointTrajectoryPoints.