The way I understand it, you and PAwoodbutcher are both right. I re-read your OP and it doesn't sound like you have ridge vents....just gable vents.
Everything you did this summer is gonna help tremendously....you know that already....IMO, for the best results, I would cut in a continuos ridge vent and seal off the gable vents.
I just did my folks' roof. They were having the same exact problems (same style house etc...). I did everything you you mentioned short of adding insulation....we cut in the ridge vent and sealed the gables....went from light gray to black shingles, with the thought that the dark shingles will melt the ice better on those 'thaw' days we get once or twice a winter!
Like you, I am very curious to see what will happen this winter!! I still have a couple more options if there isn't a sign of significant improvement.
It's also my belief that where are, there's always going to be ice on the roof.
You won't have long to wait!!....2nd week of November last year we got snow and it lasted 'till March:sad:
I always thought it was your not supposed to use gable and ridge vent together because it can cause negative pressure and suck rain into the attic. Either way im trying to make the best with what i have atm. And yeah the nails are pretty rusty on the bottom, especially down near the eaves.