Hello, Waterrock.
First, what constitutes legitimate metonymy? Well, if someone I know is heading to the store and I need some milk, I can give that person some money and ask him to pick up some milk for me. When I receive the milk, I can reasonably say that I bought the milk, even though I didn't physically make the purchase at the store. This would be an example of metonymy.
On the other hand, if someone gives me some money, I decide that I don't want it and I give it back to him, and then he decides, without any input from me, to use the mony to buy a CD player, would it be reasonable to say that I bought the CD player? OF COURSE NOT! This second example is comparable to the claim you're making about Acts' indication that Judas bought the field. It makes no sense to label it a metonymical account of what Matthew says.
Second, in these very same texts, there are two other inconsistencies (how Judas died and the reason for naming the field), which also add to the lack of credibility of these passages.
Third, Acts is a sequel to Luke. When reading both Luke and Acts together, we find that they know nothing of the return of the money (as Matthew claims). The money doesn't leave Judas' hands in the Luke/Acts story until Judas buys the field. I know you refer to this as an "argument from silence," but when considering the silence around Judas returning the money along with the claim that Judas bought the field, it makes a pretty good picture that Judas himself bought the field.
When taking all three of these points together, there is no doubt in my mind that what we have in these accounts really IS contradictory. Your attempt to reconcile this is nothing more than wishful thinking.
Take care......
