The Russian attorney was never interviewed by Mueller because Mueller KNOWS she was an agent of the Obama administration, allowed into the U.S. by Obama for the sole purpose of setting Trump up for their BOGUS collusion narrative.
It appears to me there WAS collusion - consider for a moment the two Russians who appear to have colluded with the DNC and with Hillary Clinton’s 2016 campaign:
....produced by Fusion GPS, which was paid for... by...Hillary’s campaign and the DNC...
“3 – Two Russians at the meeting evidenced a larger relationship with Fusion GPS and the controversial firm’s co-founder Glenn Simpson. The Russia collusion conspiracy theory was sparked by the discredited dossier produced by Fusion GPS, which was paid for its anti-Trump work by Trump’s primary political opponents, namely Hillary Clinton’s 2016 presidential campaign and the Democratic National Committee (DNC) via the Perkins Coie law firm.”