There actually is an explanation for what happened to Ash's dad, but it doesn't happen in any other media other than a novel written by the show's main writer. There's an entry for it in Pokemon's Tear Jerker page on TVTropes. Apparently Delia met someone, they fell in love, married at age eighteen, and he ran off onto his journey. To top it all off her mother had just died and Delia was left with a restaurant and a newborn. He's been missing ever since and has achieved little to nothing. Ash's mother has exaggerated his father's legacy, not wanting to ruin Ash's perception of him. He was a mediocre trainer. Those same novels also claim that Brock's mother married multiple times, but the fathers kept running away, leaving her with many kids and running away herself, and that Brock is so love struck with every girl he meets because he wants someone to take care of his family. It's pretty grim.While I do like Suede's review better, you bring up some good points as well, and since you're reviewing the 5th generation movies, are you by chance gonna review the Keldeo movie? Because I consider it one of the better Pokemon movies.
I'm going to review all of them eventually. Also, you apparently haven't been seeing my responses on Patreon, or if you have you haven't said anything. You can request a review anywhere, it doesn't matter if you do it there or here or on my YouTube channel.
Actually in the original Molly's mother left Spencer and in the credits there is a scene with her coming back, surprised you didn't say anything about that, her coming back is just as unexplained in both versions but you know for sure she's still alive in the Japanese one.