Ask your dentist what that sign means. To my knowledge, the bonding materials used pose no threat to your health. Unless they were talking about the metal amalgam material.
If it's not causing you problems, and the dentist said it looks ok on examination and on x-ray, then it's ok to leave it alone. The canines are typically a little darker than the teeth next to them.
If you a re experiencing pain, you should address the problem now. It will never get any easier or any less expensive than it is right now.
Structurally it should be fine. I'm sure that it wouldn't be damaged if sectioned by the lab. Sectioning the bridge should not affect the long term success of the bridge and crowns.
I do understand what you are saying. Every case is individual of the next, so you may be best off seeing a periodontist to evaluate how best to treat that area.
It's hard to tell without seeing you, but sounds like normal anatomy. To be sure have it looked at by a dentist or oral surgeon.
Without seeing you for an exam, it sounds like what you said, an irritation. If it doesn't get better or gets worse you should see a dentist or an oral surgeon.
IF you're a candidate, you will most certainly have better results with professional whitening. You risk extreme sensitivity with professional whitening, other than that, it's safe.
You could have a sensitivity to cinnamon or mint, inform your dentist of the occurrences to make sure it is nothing else.
A fistula is not normal and should have been dealt with at your earlier appointment. The surgery does have a good chance of being successful. Good luck.