Enable the auto keying record button in the timeline window, go to the interpolated keyframe, enter edit mode, select all and press Ctrl C, move to an empty frame, press Ctrl V, select this newly created keyframe and move it back to the interpolated one, overwriting it.
If you don't know which is the interpolated keyframe repeat the operation on both.
Here are a few tips to follow for the ⇧ Shift + Ctrl + E shortcut to work:
Make sure you're in Edit Mode
Select the keyframes you want to interpolate
Select the layer that you want to interpolate (make sure it shows as orange)
Click on the timeline so the cursor lies between the two keyframes for interpolation
Make sure your mouse is hovering over the 2D ...
In the Dope Sheet you can select one or more keyframes, right click on it and select the interpolation of the segments following the selected keyframes.
You can choose between ease in, ease out, ease in and out, and then choose between various interpolation rates (linear, sinusoidal, quadratic, cubic, ....., bounce, overshoot, elastic).
In 2D grease pencil ...
This is not a "full" solution, but it works and i think the "logic" is a bit simpler than yours and easier to understand.
So first there is this "animate object"-subprogram, which takes the object, start vector and an end vector, start frame and a duration and it does what it's called: it animates the object from point a to b ...