It's ambiguous in Irish and not grammatically correct, but you could hear it in common speech. The word "mar" is a preposition and so it cannot be followed by a clause without some other word. So 1. Ólann sé mar go n-ólann sí = because 2. Ólann sé mar a ólann sí = just like
Not exactly - "cosúil le" or "ar nós" can only be followed by nouns or noun phrases. So "ólann sé cosúil le capall" = he drinks like a horse. But you could say "ólann sé mar a ólann sí" = he drinks like her. And strictly the exercise question should be "ólann sé mar go n-ólann sí = he drinks because she drinks. The "a" or "go" is really required with the "mar", but admittedly you won't always hear it.