Thanks for the update. For some reason I thought the oil cooler replacement on the 6.4L was less involved than that but I may be thinking of the 6.7L. I believe the Ford recommended flushes are to remove any scale build up in the system as well as the casting sand so I think it would be worthwhile to do even if you have someone else replace the cooler.
I truly am glad I got away from this type of scenario. I loved my Ford trucks but it was exactly this type of expensive repair that drove me away from them. And sadly there's so many more common things that they are prone to.
In any case I wish you well with the repair. Please let us know how it turns out.