When and why is right to left writing used?
In the past Chinese characters were written from top to bottom and from right to left. Horizontal right to left is not that common even in ancient times. The top scroll in a couplet is one case which might be the one you see at the top of a gate. The one you see on vehicles may not be the case. I think it's just on the right side from head to tail of the vehicle which makes it right to left.
Of course not, or else it would confuse the reader.
Not for most adults, you just need to get used to it. 


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