Hopefully, it looks like flexure was the culprit.
2 days ago, I reverted back to the piggy-back installation and did a 4 minutes exposure with autoguiding enabled. I no longer had elongated stars in my images. However, with this configuration, I was unable to balance the scope in DEC due to the off-set of the stock CGEM saddle (i.e. the center of the saddle is off-set 0.25″ from the center of the DEC axis). I therefore, used parts of the tandem hardware to create a X/Y counterweight system to be fixed under the main OTA. Here a picture.
