The FBI sends investigators abroad all the time so if it is true that they have not interviewed our favorite Russian lawyer that is puzzling.
As to the substance of the fateful meeting between Don Trump Jr. and the Russian with dirt on Hillary, my thoughts are here. The gist: her "dirt" was prosecutors files making the case that Hillary and Obama had been duped (or bought?!?) when they decided to believe the Magnitsky supporters rather than Team Putin. Tump Jr's. record promoter friend knows a snooze-fest when he hears it so he did his record promoter thing and over-hyped it. Still, Jr. played along, so if - IF! - Mueller is taking the Russian collusion angle seriously he ought to be sorting this out.