Yes "Legal matters" could be a useful category, leaving "Home & Garden" to deal with building etc (although the two categories are likely to overlap at times). Posters should be encouraged to indicate which country they live in.
For editing, see "Repeat posts" below.
Yes, people keep asking legal questions in other categories, and a law category (or perhaps a law and morality category to make it more wide ranging, and subject to opinion as well) would help bring those questions together.