You need to have good relationship with KPD by the inter-relations policy, and either Levi's KPD boost. Then when the Thälmann's scandal happen (by choosing the 'International Contacts' and 'Embed a Spy in the Comintern' choice , switch to either Levi or Rosenfeld as advisor, which grant you the option to keep the scandal hidden (do NOT reveal the scandal) after which you can use the foreign ministry to see if the Soviet know about the affair by the 'foreign relations' choice, the KPD then will have a conference, if above choices are followed, do and let the KPD elect the reconciliators to leadership, there's also stuff about apologizing for the action of 1919 but I didn't tried that bcs of fast reading LOL
You have a tiny window of opportunity to do it before the game reaches 1930. So do the thing at the very first 6 months. Then if you want to grab that other achievement. Just try and RNG to get to the influence the Center event.