I have always kept my ekkies on a primarily fresh fruit and veggies diet, with maybe a 5% allowance for seeds. Through observation of their eating habits, preferences, and creative adjustments, I've gotten them to a point where their diets are balanced and complete.
There are many variables, however, when it comes to tracking down the causes of any dietary issues when it comes to eclectus parrots. In your case, given that Montego at least is benefitting from the new diet, it looks as though he was either suffering from a food sensitivity/allergy that has been alleviated by the elimination of that particular food from his daily meals, or he was being picky enough that he was suffering from a nutritional deficiency of some sort.
Some ekkie owners have reported good results while using organic pellets formulated for eclectic parrots, so if you are having some success with the new diet I'd say go with it. But watch closely. Sometimes there is a build up before toe-tapping or wing-flipping set in. Hopefully, neither of these happens and you can maintain them on their current diet.
As for Winston, his failing to re-feather (yet) could be due to any number of reasons. First off, how long was he plucking or chewing? Because it is possible that his follicles have been damaged if it was going on long enough. Or if he was pulling violently enough. You also have to ask yourself whether he has stopped plucking as Montego apparently has. Sometimes plucking can start off with a medical cause, and then evolve, through repetition, into something more behavioral. Habitual.
Heck, there's even the possibility that there was an entirely separate topical issue going on. Less likely given Montego's improvement, but it can't be discounted as a possibility, either.
Is there at least any indication that the itching has ceased?
Btw, what subspecies of eclectus are they?