Indy's true character arc in Indiana Jones and the Dial of Destiny is more hidden, but once reflected upon it actually raised my opinion of the movie greatly.
Marion's grief was not the reason for their separation, but Indy's depression and numbness caused by the death of his son was. Indy in DoD is withdrawn from the world around him. It is likely he closed himself off from Marion and was blaming his own depression and grief issues on her to push her away, because like with most cases with men's mental health issues, Indy is a strong man, and his sadness isn't the problem.
At the beginning of the movie when we see Indy's apartment, it's mess. Indy is passed out drunk and unaware in his chair. As we find out later, there is barely any food there, so Indy is not taking care of himself. When the neighbors start playing loud music and we see angry Indy, he is not even aware it is "Moon Day". One of the biggest historical events of all time is happening, and Indy isn't even aware of it.
Then we get to the university, none of the students show an interest in what Indy has to say, and his colleagues can't wait to get him out of the door to his retirement. But, when Helena shows up and actually shows an interest in Indy's teachings, it starts to break him from his cycle of depression and gives him something worth exploring in his life again.
One of the criticisms I've read about DoD is: why is Indy even after the Dial? He really isn't. He's after the adventure again. Feeling alive and having something in his life that makes it worth living again. This is hinted at when Sallah drops him off at the airport, and Indy comments that the days of adventure are long behind them.
It isn't until the evening scene on the boat where Helena really begins to understand Indy though. She asks him what he would change if he could go back in time and Indy's answer was: "I'd stop my son from enlisting". Helena begins to care more for Indy at this point and can now see how his depression and grief has affected him and the great Indiana Jones she knew as a kid.
When they travel back in time towards the end, Indy wants to stay, and in Helena's mind, this is Indy fully giving into his depression and taking himself out of the world he belongs in. Indy is shot and is in a time where he would likely die very soon because of the lack of modern medicine. Indy wanting to stay is basically Indy wanting to kill himself, and Helena won't let that happen at any cost. She pleads with and eventually knocks out Indy and they navigate back to present day, with Helena knowing the only way Indy will ever be truly happy again is to reconcile with his wife, and get out of the cycle of depression he has been in.
Back in New York, Helena explains everything to Marion, and she now fully understands why Indy has been cold and withdrawn in their relationship, the grief is still affecting him, even more so than her, and Indy's lack of opening up about his feelings is what caused their strain. That is why the first thing Marion does is ask him: "Are you back?". She asks it several times to confirm he is able to break his cycle and try living again.
And then in the final shot, we see him pick the hat up again. Indy is ready to break the cycle. Indiana Jones is ready for adventure again.
Indiana Jones and the Dial of Destiny is more of a character study for men's mental health, and how grief can affect you and the relationships around you, and how that grief can eventually be overcome with the help of others and by choosing to keep going. Once I realized this hidden story, it made me look at the movie in a different light and appreciate it even more.
