I'm not a DL expert, but here goes.
1) I don't really know. But I you do end up doing summer, July usually is cooler than August, sometimes by a lot. There often is fog in the mornings that takes some time to burn off. That is even more common in June.
2) I think there is enough visual interest even without knowing who all the characters are. For Mickey and Minnie you will want to visit ToonTown. Winnie the Pooh and friends are in the back beyon Splash Mountain, where Bear Country Jamboree used to be.
3) I don't know anything about the Marriott. Sorry. I understand about wanting to use points, though.
4) Since I don't know where the Marriott is, I don't know how practical it is to go carless. If the hotel is close to the resort, you should be okay with airport-to-hotel shuttles.
5) I haven't been there much since Chuggington came on the air, but I've never seen any Chuggington stuff. There are several trains, though. The DL Railroad, of course, which still takes its journey through the Grand Canyon, present and past. The little circus train from Dumbo in Fantasyland. And a cartoonish train car that is stationary in ToonTown. Oh, yeah, and the monorail!
Star Tours is still there and better than ever. There are multiple journeys now, with various storylines to go with them.
I don't know if you're doing California Adventure, too. Most of the Pixar stuff is over there (the exceptions are the Toy Story ride and the Finding Nemo submarines, both in Tomorrowland).
So much of Disneyland is just good storytelling, whether there's a movie/show connection or not. (How long until they make a movie based on the Jungle Cruise?)
Have fun!
DS '04 "Boogaboo"
DD '08 "Lilybear"