Hm, while I am personally very impressed by the pin (I would not have realized it, if you had not told me), I do not think that it would have worked that well. If I get it right, he would have needed 5 copper every turn, that he can pass to you via masquerade. In combination with you giving him curses via ambassador, it feels like that would slow down the scrying pools too much. Also that means he needs 5 buys for copper every turn, so he needs additional actions for squires each turn.
I guess if you had had some scrying pools, you would have finished well before he had the pin ready.
So, my personal answers:
1) no
2) as soon as you get 4 coins after the double amb
3) don't think this would be necessary at all, if you go for scrying pool.
4) I would rather aim for a quick ending, kindda as you did, than at the pin.