First off, Seiko's stated accuracy for their 6R is actually worse than what you've listed.
Source: Seiko's own technical specs say +25 - 15 seconds per day
Your post is interesting but I think it is fundamentally flawed. The "stated" accuracy of these movements straight from the factory isn't the only factor to consider. Many of the brands that use ETA, Sellita, STP, etc will perform modifications, service, and add custom parts to these movements to improve the accuracy and durability of the movement out of the factory. Brands like Sinn and Hamilton do this to varying degrees. It also serves as a second level of QC that doesn't happen with Seiko.
It's like comparing a stock engine straight from the factory to one that's been tuned and slightly modified by a mechanic. They might have the same listed horsepower and be the same engine, but one will outperform the other. Grades are also a thing to consider ex: https://calibercorner.com/sellita-caliber-sw220-1/ Sinn for example uses the SSW-220-1. You might google that and say the accuracy is up to +/- 30 s/d. However that movement has 4 grades, and Sinn uses the "top" grade of movement with a stated accuracy of +/- 4 s/d.
Save screenshots of all the tracking staus pages in case they become unavailable.
Open and shut chargeback. CC company shouldn't hassle you at all, you have 1 tracking number with DHL saying delivered and the subcontractor saying damaged - not delivered. Clear chain of custody to a non-deliver. Plus it's only $250.
Just expect Jomashop to blacklist you. Not that I'd expect you to order from them again...