Great first post. Interesting observations for sure.
Spotted Bass are native to the Darby, especially the lower end. Not surprised in the least with Juvenile Musky showing up too, again, especially in the lower stretches of the Creek.
On to the Trout. It's highly likely someone, at some point, attempted to privately stock them in the Little Darby. It's also possible (though much less likely) the ODNR stocked them at one point but didn't tell anyone (they do this from time to time for various reasons). Im not completely surprised that someone, at some point, caught a few out of the (Big) Darby - Especially right at the confluence. That said, nowadays they have to be incredibly rare, if not completely non-existent in the BIG Darby.
I'd love to be proven wrong.