I'm so sorry you're so unwell. I know many people have struggled with symptoms prior to being diagnosed.
My husband's tumors are non functional so he doesn't have any syndrome, so I'm afraid I can't help with anything related to that. But I do know that most people with carcinoid syndrome do see evidence of that in the 24 hour urine test, so the fact that yours was negative would seem to be good news.
There is a net specific pet scan, gallium 68, that covers the entire body. If there are any net tumors with SSTR receptors, that scan will pick them up. While some nets don't have receptors so they don't light up, the majority do have the receptors making this pet scan a particularly helpful tool.
I understand your stress and anxiety while looking for an answer. It's difficult to address any what if questions without a diagnosis, however. In the meantime, I would suggest that you focus on what you can control to improve your overall health. Good underlying health is so important to he able to help your body fight any disease. Try to focus on improving your basic health by eating right, exercising and minimize alcohol intake. Maybe also seek out a counselor to help you deal with the stress that you are under since stress does have a big impact on your health.
I pray that you find an answer amd start feeling better soon!