Nick, I'm of the opinion that you may still have a condensation problem, even with the venting that's been installed, but from the added information I'd also suspect a roof leak of some kind such as a flashing problem around that dormer.
The only way I can think of to possibly get a visual of the condition and installation of the insulation would be by removing a section of the soffit vent, but this might also not reveal a thing. As to the possibility of a roof or dormer flashing leak, this often takes a trained eye to find the problem, especially without any attic, and can be quite tricky to locate.
Personally, as the home is new and you're already aware the builder has cut corners which not only caused problems, but were also not within code, I would first be looking to the builder to determine the cause and for the correction of the issue(s)if they are due to buider error. Was the builder also the one to retrofit the venting? Is the builder aware of the situation, and have they been good to get along with, prompt to take care of problems?
If the builder has not been made aware, you do need to inform them and give them the opportunity to make good on this first. If they already know, and you feel that they are attempting to "ignore you hoping you'll go away," then I would contact the city or county office which issed your building permit for advice on having an independent party assess the nature of the problem.
If the cause is determined to be error on the builder's part, an independent report on the home will be to your advantage, should you require "leverage" to help get this resolved; and it should be resolved soon, before it causes further damage. I really feel no matter who's going to wind up taking care of the repairs, at this point I'd be getting someone out there to take a good, long look at this to determine the exact cause.
I'm going to place a new post asking that some of the other people "in the know" here chime in if they have anything to add that will help you, and let's see what other thoughts come to light.