"Robaron en su casa" and "robaron su casa" may mean the same. Yet, for me, it's not only a matter of personal preference. In this case, I was thinking of a thief that entered a house and only took some valuable things. If I say "robaron su casa", I probably have in mind they took everything (not only valuables, but also furniture, clothes and anything else). It's not necessarily so, but that's how it came to me when I wrote the examples.
As for "robaron a su casa", it would make no sense. The house is not a person and there is no other case in which anyone would use this preposition.
"Les maltrataban" is "leísmo"; it is used in some places, but not where I live. You have a Direct Object, and the necessary pronoun is "los".
Don't worry, you're coping fine. It's only a matter of regular practice to assimilate most nuances and constructions. Keep asking until it's all clear, and if you stumble upon it again, ask again. That's the only way I know to learn things.