If all else fails, and you do not want to re-use the seacock anyway...
Get a micro tool like a Dremel (go for one of the corded ones, the rechargeables are more useless and frustrationg then anything else. Depending on how much room you have, drill some holes into the collar and body of the seacock to asses how far inside it the thread of the through-hull runs. Cut it off with a hacksaw (if it fits) or the dremel cut-off wheels just above this point.
Now use the cut off wheel(s) to slit the remainded of the collar off the threaded through hull. Delicate, but eminently doable. Get a little bity cold chisel and small hammer and just tap gently to seperate the remains of the seacock fromt the through-hull (Aim the chisel so it will not mash the threads on the through-hull should it make contact). You will find it works quite well if all the chemical release agents and much swearing and bruising of knuckles has failed.