If I understand your objective correctly (that’s a big if, but I’m confident) then crop: source isn’t really what you want to use at the start.
In your case, you DO want crop, but crop rectangle - applied as a filter - after the Size, Position, Rotate (SPR) filter. The easiest way is going to be to import your video to V1, add the SPR and the crop rectangle (in that order), then do your SPR keyframing, moving it around as necessary. Once you are happy with your work, only then copy that to V2 and apply the mirror filter and voila!
Here is an example created exactly as I described above. Just unzip the mlt and the video to the same folder, then open in Shotcut to see the how it works (there are keyframes on the SPR moving the video back and forth).
There might be another way to do this, but in the meantime this works and isn’t difficult to implement (assuming I did understand you correctly!).
Mirror.zip (1.2 MB)
Output: