Can a subsidiary buy a company?

yes - it depends on the corporate structure

(there is no reason why "marvel comics" could not operate as a financially autonomous entity

- i.e it would operate just like any other company does with shareholders owning its stock,. The company decides to buy another company it buys, without necessarily having to refer to the stockholders. But in "marvels" case disney would own the stock, not shareholders

But you have to remember - with "marvel" its very likely disney would have their own people as directors on "marvels" board of directors (they dont have to but its very likely) In this case they would have to vote to approve the purchase of a company by "marvel"

i.e Marvel would still be doing the actual buying not disney, but they would be doing it with disney's approval)

yes it can

Any level *can* buy another company. That said, A decision that large would almost always need to be reviewed and approved by the parent company. It would be very rare that a subsidiary would have the independence to do this all on their own.