Good points, except the disciples couldn't go to the lost tribes as they had no idea where they were! Plus they were forbidden to go to any cities in Samaria. I think if you check the Greek word for "lost" you will see that its definition is more towards "the perishing", so in fact they were sent out to the Jews who lived around there.
They knew exactly where they were....they had only been lost to scripture. The first century historian, Josephus, mentions in his "Antiquities" Book XI, Chapter V, Paragraph 2 that there were only two tribes left in Asia (minor) subject to the Romans, the other ten were beyond the Euphrates and were beyond counting.....they were so many!
Also please see this.... Babylonian Jewry
Don't you think that divinely appointed Apostles would be able to find these folks.....if they had been told to do so?