From your diagram, it seems to me that the only venting you have right now is at the gable-end(s). This is the 'high point' of the roof. So all you need is air to enter the attic from a 'lower' point in order to 'push' out any warm air that's risen.
First, you need to do what you can to seal off the attic from the rest of the house. I'm going to assume you've got that under control. The only thing I would consider is putting in soffit vents on BOTH ENDS of your // Adding roof vents won't help in 'pushing' any warm air out. And using inside air isn't the answer either because all I can envision is warm air heating the 'valley' and the water freezing, thus, exacerbating your current problem.
So, from what I see, soffit vents on both 'ends' is the best solution. My best to ya and hope this helps. Hopefully, someone else will have a better / different answer.