When ever I get one of those &*$&^ $ $( screws I take an old screw driver
heat it cheery red and place it in the screw head this transfers the heat to the screw also try transmission fluid it may take several time but it will loosen
Try leaving it vertical all night with tons of WD40 impregnated and then go for it with a big screwdriver that fits perfectly; you may end up unscrewing the return spring tube though!