That picture is clear, and I don't see anything in it that keeps you from pulling the blade boss off with a puller by smacking the end of the puller screw. However if it is really stuck, a propane or oxy torch on the body of the blade boss (definitely not on the crankshaft) should do the trick. Unless it comes off fairly quickly though, heat will be conducted to the crankshaft and will destroy the oil seal, which you would then have to replace. Incidentally you were correct in calling the longitudinal slots in the crankshaft and blade boss keyways. It is a bit unusual to use two of them; perhaps they had a problem with keys shearing in some application and that was their response.
A heat gun does not supply enough heat to be any use for this job, probably by a factor of ten. It also applies the heat over a wide area rather than right where you want it, and this would be a bad idea.
