Only you can answer this question because we are all so different with fibro. I know that I have never let fibro run my life. If I liked my job, like you do, I would find a way to do it. You said it's a laid back job and that's wonderful. You can use your creative abilities and that's wonderful, too. So, I think I would look at the aspects of the job that could possibly bring on more pain and think of ways to work around it. We might even be able to help you there.
It sounds more like a desk-type job so you could print out the gentle stretching exercises and do those while sitting down and working. You should get up a walk for a couple of minutes whenever possible. You could take a Bed Buddy (you can get these at Walgreens and other places) with you and pop it in the microwave at work (hopefully you have one) and it gives off moist heat to help with shoulders and neck.
If you pack a lunch, pack things like salad, left-over meat, fresh fruit, etc. Keep carbohydrates and sweets to a minimum because these can raise your blood sugar and then it crashes in the afternoon and you will be sleepy. Drink plenty of water and avoid sodas. I love hot tea so that's what I drink in the morning....no sugar.
This is all I can think of off the top of my head. If you have an area where you need suggestions, explain it to us and we can put our heads together and hopefully find ways for you to work around it.
It sounds like you really want to keep this position so I would go for it. You will never know if you could do it if you don't try. If it doesn't work out, then you can sit down with your boss and re-discuss the other positions in the company.
Sherrine