You ask three questions in one in your question about Shelley's Frankenstein. I'll answer one for you.
If the novel were a Shakespearean tragedy, which it isn't, I would choose the moment that Victor impulsively destroys what he has already created of the female monster. That certainly is the moment that signals his doom.
But since it's a Gothic novel from the 19th century, I'll suggest that the climax occurs when the monster kills Elizabeth. This is the moment when the monster wins and Victor loses. That's when the conflict is decided, the highest point of tension, the point at which Victor is truly destroyed, etc.