1. I met his wife ______________ : (in 2004, last night, three months ago, just now)
All the answers are possible.
2. She has ___________ finished the book. : (already, just).
Both the answers are possible.
"Just now" seems to go with the present, why does it goes with the past?
Both just and just now can have a number of slightly different meanings; the context normally makes things clear.
She (has) just finished the book. = a fairly short time ago, or even at the moment of speaking. The present perfect is more normal in BrE, the past simple common in AmE. He arrived just as I was leaving. = at precisely the same moment. Sorry, I'm working just now. = at this present moment. I saw her just now. = a short time ago.
There's also a dialectal variant that has nothing to do with the tense of its ver b(or at least - one that I wasn't aware of when a friend of mine used to say 'Cheerio just now' at the end of a conversation. (She was Scottish; I think it meant '...but I'll see you again soon'.)