I think you might be misunderstanding the situation. Both Amazon and Ingram Spark are perfectly capable of printing the book as designed, as is evidenced by the numerous proof copies I'm sitting on. They both offer full bleed and are happy to allow cover elements to bleed off the page. So there were no technical limitations here.

The problem is with Amazon's reading of the requirement to "make sure that all elements intended to be viewable appear at least 0.716in (18.175mm) away from the outside edges."

While Ingram Spark understood that this part of the design wasn't intended to be viewable and were happy to go ahead and approve the book, Amazon were arguing that it needed to be viewable, despite my explanation to the contrary.

If this was purely a technical issue it wouldn't be a problem. The fact that they can and have printed the design but somebody has decided (wrongly) that it contravenes their guidlines is the problem.