You did mention that some of those shot down by these aircraft were already damaged by their escorting fighters, making the shoot downs plausible.

Though the shoot down claims could be exaggerated, it's not entirely impossible for a gunner in a Catalina to shoot down another aircraft.
My dad told me once "Once the bullets start flying, there's no such thing as a bullet with your name on it. All of them are labeled To Whom It May Concern."

I believe him not just because he's my dad. He trained in the SAS Jungle Warfare School in Malaysia and the US Army's Special Warfare School in Fort Bragg, saw action in Indonesia and Mindanao. The Huey he was on once was shot down and I had to visit him as a kid at V. Luna military hospital.

He, along with my aunts and uncles, related stories of their refugee column being strafed by Japanese planes. Everybody just ran looking for cover because nobody can tell whether they were going to get hit or not.