re: What was Muller investigating?

I don't think that there's anything publically known that draws a straight line.

Cohen is likely a subject of the investigation likely with regard to his part in the Artemenko peace plan/sanctions relief, trying to line up Kremlin supportVTB bank funding for Trump Tower Moscow during the campaign, etc. Also, Mueller is generally looking into foreign money in the campaign (UAE dollars, Russian $ to the NRA, etc). Cohen has worked as much as a deal-facilitator as a lawyer for Trump and his wife is Ukrainian. He's bought blocks of condos in Trump Tower and resold them. There have been questions for years about Russians buying Trump condos for purposes of money laundering. The LLC he used to pay stormy might be one of many questionable ones coming up for Mueller.