"We went to the sea and mountain on the weekend."
The correct solution shows that "週まつにうみや山に行きました。" is accepted, however, when you're stuck with that exercise of having to pick words using different blocks, two にs aren't available.
Is it relevant that sea and mountain are translated in the same order? Seems as though unless it says "we went to the sea and THEN the mountain" that the two would be interchangeable, but apparently not?
Japanese does not use pronouns as often as English, so it is better to get used to leaving them out. It can be hard to get used to at first and often you must infer the pronoun based on context. : )
This sentence could be changed to 私たちは週末に at the beginning, but sometimes direct translations are wrong or feel clunky when spoken.
No, the subject is understood to be first person unless otherwise indicated.