Both Liebknecht and Luxemburg probably would have been sidelined in the KPD during the faction fights of the early-to-mid 1920s. As she was in favour of intra-party democracy, I could see Luxemburg becoming part of the Brandler/Thalheimer faction of the KPD. Luxemburg's KPD career probably would have been over when the ultra left-wingers took over (1923/24). At that time, there were still rivalling factions within the leading troika: Ruth Fischer and Arkadi Maslov were the German supporters of Zinoviev and Kameniev, whereas Thälmann was very much Stalin's man from the beginning. There were also some Communists who were still willing to cooperate within the parliamentarian framework. Some, like Paul Levi, eventually joined the SPD later. So I think the only thinkable ways of Luxemburg getting elected somehow would be either with her becoming part of the USPD in the early 1920s (and eventually the SPD later, which would have made her the leading figure of the left-wing there), or as part of a KPD more independent from Moscow.
August Bebel famously once described Rosa Luxemburg as "the only real tough guy in the SPD". Maybe she could have been some sort of missing link the OTL KPD and SPD didn't have.