Did Rimuru Absorb Charybdis?

Rimuru obtained this Extra Skill when he absorbed Charybdis' Core. It was later integrated into the Law Manipulation aspect of Ultimate Skill Uriel.

Who defeated Charybdis?

The Odyssey

Odysseus faced both Charybdis and Scylla while rowing through a narrow channel. He ordered his men to avoid Charybdis, thus forcing them to pass near Scylla, which resulted in the deaths of six of his men. Later, stranded on a raft, Odysseus was swept back through the strait and passed near Charybdis.

Can Rimuru defeat Milim?

According to the light novel, Rimuru eventually grows stronger and surpasses Milim. During the Great War Arc, the Demon Lord Rimuru unlocks True Dragon form, thus surpassing Milim.