I don't think 6E should be either Shadowdark or Daggerheart (even though I believe that if 6E came out as one of those 2 rulesets people would love the new D&D). Those games exist. 6E should do something only WotC can do. That means scale and reach and broad appeal.
Imagine if 6E actually...
For a long time there were weird "rules" based on how things were licensed. My guess is that Namor was part of the FF "group" and that seems to be back in the hands of Marvel.
I wouldn't expect the PDFs to be free on their own. It just seems odd to charge for them when folks are buying the hardcover.
But you know your business best. Good luck with the campaign!
Again, this tightly integrates the two axis so that there is only one axis. And that is fine, but not what DH does. it is inherently more limited and limiting than the two axis resolution.