So Henry names two living sons Henry because . . .
I suspect he'd name a second son Edward (as a name with lots of good royal tradition) or John (after his - Henry V's that is - grandfather).
And why would the brother usurp the throne?
Or marry Margaret of Anjou?
Mad brother makes the perfect opportunity. It happens a lot. As for the name, it's just an example. Margaret of Anjou marriage is only a possibility.