You are 100% correct about the origin & purpose of the 6.0L, v8440, including the fact that Navistar should've declined. However, once it had gone completely sideways, Ford told Navistar to take a flying leap with regards to the PowerStroke-based V6 for the F-150. And that's where the beat-down started.
The Powerstroke V6 project was so far along you can actually find evidence of it in Ford parts catalogs of that time frame if you have a sharp eye. Navistar told Ford, "We have a contract to which you are bound, skippy." Ford tried to weasel their way out of it, and Navistar won a $75 million breach-of-contract suit against Ford. The PowerStroke had already cost Ford a little over $75 million in warranty claims at the time of that judgment, meaning the 6.0L had hit Ford to the tune of $150 million--and that was 6-7 years ago. Though the 6.4L PowerStroke was still a Navi engine, the damage was done and their longtime relationship was dissolved during the 6.4L era. During the dissolution of that partnership, Navistar was released from further liability in regards to the engines they'd supplied to Ford including the 6.4L they were still supplying, which meant Ford was buying "crate liabilities" for all intents and purposes. Ford simply could not ignore the diesel market until they could release their homegrown "Scorpion" 6.7L diesel for 2011, so they rolled the dice. Luckily for Ford, the 6.4L was a vast improvement over the 6.0L. Ironically, much of what made the 6.4L engine better grew out of the ill-fated PS V6.
Fast forward a few years to 2013, and the class-action lawsuit against Ford by 6.0L is settled. Navistar's financial liability for that lawsuit: $0. It won't cost them a dime since Ford released them years ago.
Navistar, never ones to rest on their laurels, after finally making a pretty-good light-diesel duty that they couldn't sell, then proceeded to shoot themselves in both feet with the heavy-duty MaxForce, an engine so bad the local International dealer is rapidly circling the drain and International/Navistar itself might not be around much longer afterwards.
If International/Navistar had half the skill in their engineering department as they do in legal, they'd rule the HD diesel world.