Am I correct in my action, considering that no natural discharge, i.e., semen, was emitted, and that is because I doubted it was semen and because the situation was not one that aroused lust, despite the whispers of Satan that my prayers will not be accepted?

What you have read, that the Shafi'is hold that one who doubts about what exits from them may choose, and assign it the ruling of one of the things about which they doubt, is correct and is the relied-upon opinion. Therefore, what you did, by considering what exited to be vaginal discharge and not performing ghusl (definition), is correct, and there is no blame upon you for it. It is advisable to continue ignoring the whispers (waswas) and turning away from them.