ジャマ is “hindrance; obstacle; nuisance”. (I learned this one back when I watched Ojamajo Doremi years ago, and it comes up a lot in that anime.) So, the verb する (to do) applies to the object “interference of Dark Kingdom”.
I’m glad you asked, as this was one of those things that I passed over because I understood the gist of the overall sentence.
According to this N1 grammar page, 「 誰であろうと」 means “whoever”. I’ll want to look into this some more (to find out how exactly it works out to that meaning), but there it is.
It is indeed. It may or may not be (although I was at first certain it was). See the WordReference Forums link below.
Edit: Good discussion here as well: