Is Rika completely gone? How did Okkotsu retained his power?

that's my question too, but based on what happened i think he's strong too without Rika, also Gojo said that yuta might surpass him.. so yea he's gifted

I guess we'll just have to wait for Okkotsu to appear in the main series to answer all our questions

yes

He's strong not because of Rika but because he's the one who created rika curse int he first place. So Rika original power was Okkotsu power from the beginning. He will probably learn some new technique.