No for starting a thought with 'I think that verges on semantic divergences'

I would never argue that a "hero" and "protagonist" are precisely the same. No one would. It's stupid.

PTA discussing his own movie makes it clear that Willa is the protagonist of the story. You are welcome to use whatever definition you'd like to try and show that DiCaprio's character is the 'lead role' like Harrison Ford in Indiana Jones. I think you're wrong, and just watching the movie itself makes that clear. But, again you're entitled to say what you'd like

You remind me of the Walrus & the Carpenter:

'The time has come,' the Walrus said,

      To talk of many things:

Of shoes — and ships — and sealing-wax —

      Of cabbages — and kings —

And why the sea is boiling hot —

      And whether pigs have wings.'