Nice researching of the source codes. Everything you said was exactly right.
Right, sometime in 2014-2015 Google Analytics instructed anyone who wanted to remarket using Analytics data ... to replace ga.js with the 3rd party DoubleClick dc.js. Pretty sure the actual internal script is identical once loaded.
The thing is though, that dc.js modification was of course tested by us at that time in 2014-2015 and maintained. It had been functioning fine from 2014-2015 until now, September 2016.
Right, a conversion is fired upon clicking 'Add to Cart'. Not only is this an introductory-level $2 conversion that can help measure AdWords performance and then remarket-to this group (who did not also complete a $200 purchase) .... but it is primarily a click-fraud filter. Click-frauders don't tend to click an 'Add to Cart' button.
We have not added GA to the Thank You page. Only AdWords conversion tracking. AdWords conversion tracking, along with Ecommerce, that gets passed through Free Guest Checkout fine. That, along with Ecommerce, does not get passed through the PayPal step anymore since September 2016.
The analytics on the Thank You page referenced was the E-junkie version, which possibly might have changed? Someone said they actually saw a different version being used by E-junkie now within a Google Tag Manager block at the top ... ever since September 2016. Maybe this is not the case, I'm not sure.
All was good for 2+ years until Friday September 23, 2016 2am MST.
Since all remains good using Free Guest Checkout for the 'free product', it would seem all our 'paid product' data is getting eaten by the new PayPal SSL configuration rather than the dc.js / ga.js mismatch or any other possible script version mismatches ... no?