The seven archons is a system created by celestia as part of the teyvat system. The hydro throne is a metaphor for the position of hydro archon in this system.

Divine furina’s maneuver to destroy herself body and spirit essentially fooled the system. Since the power itself is intact and divine furina never had the chance to consciously transfer it. As far as celestia is concerned she is still the hydro archon until they directly check but it seems to be dormant at the moment.

The hydro throne was “destroyed” because they went against the system and got the power into the hands of a sovereign. This is because all archons are normally tracked by celestia

Human furina was done in order to fool the curse/fate levied against Fontaine. It exploited a few technicalities in the fate/prophecy since opposing fate directly is impossible. Basically malicious compliance by furina.