Thank you for your last corrections. Can you check these statements, too?

Question

Thank you very much.

1)They finally arrived on an island, where once had lived the witch Sykorax and which was inhabited now by her son Caliban and Ariel, the spirit of air.

Answer 1

there between where and once. Shouldn't there be a comma like this , and. Pluralize sons and spirits because there are two of them.