I think you are overreacting. I am terrified of spiders to that extent. I wouldn't divorce my husband over a practical joke, I would tell him how I felt and move forward.

He obviously doesn't understand your fear and there is obvious communication issues. You didn't communicate with him after the incident apart from "changing the sheets" or "go away". You didn't tell him how you feel , you just packed your bags and left.

He saw you lying on the floor after the incident and he kept coming in to check on you, every time he checked on you, you told him to "go away" .

I agree with your fiance that it was an overreaction. I am not downplaying your fear, but once you realised it was a fake you had every opportunity ( after you calmed down) to communicate with him effectively and generate an adult conversation.

My husband did play a practical joke identical to that when we were dating. The difference is I told him how I felt gave an opportunity for him to apologise and he has never done it again

TALK to him