For anyone still experiencing issues - the resolution that I found to create the greatest success was by installing the "Flexify" app per Shopify recommendation. The sales skyrocketed in my remarketing campaign after making the adjustment and I have had no issues since this installation. Prior to installing Flexify remarketing sales were way down and I kept having major connectivity issues.
I would like to add, without Flexify things APPEARED to be good at times, data was being collected and everything was signalling as ALL GOOD. However, they were not and because of this sales suffered. One I installed Flexify things picked back up. The rep at Shopify told me that this was a highly recommended action and she was absolutely right at this time. Things may change in the future.
I had been using the Flexify app for a few months but I just tried associating our pixel with Shopify's catalog and it actually worked. All I did was associate it directly in Facebook Business Manager in the Shopify catalog settings. I can say without a doubt (and after giving up on the first 100 tries) this wasn't an option previously so maybe they finally changed something on their end?
Flexify did the trick and eventually, it was so easy. I actually struggled for weeks with this and tried everything. Once I found this video https://www.youtube.com/watch?v=z4kt8nyBc58 of how to set up the RSS feed with Flexify it was so easy. Good luck
I tried a couple things but once I connected to facebook through the Marketing section of Shopify admin, I had a duplicate catalog on facebook to which I was able to add the pixel. Seems too easy after all the trouble!
Only issue so far is that there are some errors in the catalog such as missing the google_product_category.