I’m a nerd who follows rights issues of the MCU for reasons I suppose, so I can answer the question in Footnote 5: the MCU started as a three studio deal between Paramount, Universal, and what would become Marvel Studios (but started as barely a Production company). Marvel lent Paramount a bag of “lesser known” heroes like Iron Man and Thor, but Universal was already holding cards at the table having held the Hulk rights for a few decades (including the Avi Arad, definitely not in the MCU film). It had to be a multi-studio deal because they wanted to make one or two movies and year and needed the resources of multiple studios (modern Disney must laugh at this a lot, though at the time the MCU movies were a huge risk because Hollywood investors only cared about Batman, Superman, and Hulk), but of course it was also because they knew they were making an Avengers level play and knew they’d need a Hulk (if for no other reason than that investor reason he was the only Marvel character a lot of old Hollywood money knew). Marvel Studios must have played both studios against each other really well to bully Paramount in keeping almost all the rights and using Paramount almost like a grey label studio (because Universal had the Hulk and they only had characters on loan) and also knew that to make a multi-studio operation work and a possible Avengers play a reality they had to negotiate Hulk rights back from Universal, knew they were in a weak position but got rights to Hulk’s appearance in movies where he’s not solo/not the title star.

So yeah, fun long story short Universal still supposedly owns the rights to Hulk solo films (with Marvel Studios approval apparently) and The Incredible Hulk is still much more of a Universal film than say Iron Man was a Paramount film (though their logo is still on it, and I’m sure they still get residuals, but it’s on Disney+ with few complications). Allegedly Universal is steamed that the MCU hasn’t wanted or needed a second solo Hulk film and now that Marvel Studios is fully owned by Disney that be so weirdly awkward and unlike Spider-Man Disney probably doesn’t won’t have a need for a solo Hulk film again. (A lot of those old investors that loved Hulk because of the 70s TV show aged out, died, or retired, including the MCU’s own Producer Perlmutter.)

I find these multi-studio dynamics fascinating because there is no movie project in history and unlikely to be one again that has touched so many different studios among its movies with Paramount, Universal, Sony, and Disney.