As far as the busted flange goes , I don't think it's worth welding - it'll always be weak , since it's cast iron .
I'd look for another for a 'parts stool' .
If you must decide to have your cast iron flange welded , I'd suggest you have your metalworker fill it up with babbet mat'l , (after it's been welded back together) so as to lend some support to your assembly .
Most piano stools I've sat on have ACME threads for the seat .... is this so for yours , rs74 ?