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It's definitely worth asking about that canonical link. If they are nice, they'll add it - and I've personally been lucky with that before. If they were "nice" enough to credit you for the blog post and link to the original source, then they should be fine with adding it.

Using Noindex is a solution to prevent any problems, but what about linking to your original (law) source? The page could be indexed and as long as you have enough good quality content on your website you should not have any trouble i think. CleverPHD makes a good point about writing some text of your own!