I have a hunch that автобусом is more often used in the west, and на автобусі in the east.
You can say even Їхати в автобусі :)
Hmm.... I've never heard "в автобусі" being used as "taking a bus" (only meaning literally, "I'm on a bus right now" - Я в автобусі)
That's true. "By bus" it's "автобусом". "На автобусі" means "in the bus"
Ідуть is not correct? Only їдуть?
іти is when you walk, їхати is when you use vehicle