You don't need to start again, clearing (trunc) the orders, invoices, hosting, domains, accounts tables and use sql (or whmcs config) to set what the next id/numbers should be - this is simpler than trying to pull selective data over.
As to tidying users - yes.
delete from tblusers_clients where there is no matching tblclients.id to client_id
then delete from tblusers where there is no matching tblusers.id to auth_user_id