3D Printing I messed up big time

You should be able to take the headcap easily, is it magnets or pieces that latch together? Since you need the turn the S hook in the neck to attach the body, you need to remove the headcap.

Edit: I misread, I understand you made the doll, but I think you should cut a headcap from the head, so you can attach the body, but also so that you can change the eyes, install eyelashes, ect. The only way I can imagine being able to turn the S hook while it's inside the head is to use a latch hook and go through the eye, but that seems a bit impractical imo

 

The wording you use leaves me with questions.
First when you say you attached the head cap how did you attach it?
Second question, are you making your own doll from scratch? or just putting together your first BJD?
Typically the head caps are meant to open and close so you can change out eyes and attach the head to the neck my S hook or some kind of latch system.

 

How is the head cap attached? The head cap needs to be removeable to work as intended. If you are in the sculpting phase you need to be able to remove it for moldmaking and if the doll is finished you need to be able to remove it to change and adjust the position of the eyes as well as to access the S-hook.
What are you making your doll out of?

 

You should be able to cut a headcap off with a piercing saw! Just make sure to wear proper respiratory protection

 

It sounds like you'll need to un-attach the headcap from the head before you can proceed. The best way to do that will depend on how you attached it in the first place.

 

depending on the neck hole you could try using screwgle. These are used with porcelain dolls that have a dome head. They basically screw into the neck hole and have a hook at the bottom to hold the elastic. It would just depend on the sizing.