sitting on the throne, in symbolic meaning

It would be sat on the throne, but yes, it can mean to reign. Usually, in the UK at least, it means to sit on the lavatory. The toilet is often euphemistically referred to as the throneroom.

So the answer to your question is yes, but it's hardly ever used because it would attract far too many giggles.

You mean the king sat on the throne for twenty years

Thanks very much!

Not dropping an aitch in extremis.