Here's my Demo: Bow Accelerations Level 1 - JHS.zip (1.4 MB)
The Bow may not be in the right spot but You will get the idea.
I noticed You have the ellipse behind the violin - and I assumed the bow should be over the violin. In this situation grouping will not work because the ellipse then be placed over the violin.
What I did in this Demo was to copy the motion path from the ellipse (in the Timeline layers), select the bow in its Timeline layer and paste the ellipse's motion path to the bow (making sure the Timeline matched the starting point of the ellipse - in this case "0").
You can tweak the motion path of the bow - especially in regards to where I have the bow located.
Alternatively if making the ellipse transparent would be OK (say a value of 65% as per your original) as a replacement simulator for your "under the violin" effect, then You could group ellipse & bow - also assuming the ellipse & bow are always positioned in the same spot in relation to each other. Once You establish the motion path for the group You could adjust the transparency of the ellipse as it passes over the violin to match your original effect.
The ellipse's motion path will not work well for a straight copy~paste to the ellipse & bow grouping (I tried it). You would be best off simply creating a new motion path for the ellipse & bow grouping (and eliminating the one for the ellipse). With just one motion path for the group you won't need to futz with separate paths for the ellipse & violin (though that would still be good solution).
Screen Shot "ellipse & bow" group - notice it is the group that has the motion path.