I neither really like taking IGG over the Witch, nor with Bishops:
Witch: When you want to rush the IGG, you have to take IGG with empty Cursepile to 3pile. I think IGG to support the Witch is fine to win the Curse-split, but I would start with a Witch. When you can play it twice, which should not be so hard, it handed out more Curses than the IGG you bought there.
Bishop: With the IGG, you hand out Curses, and of course that slows trashing of Estates/Coppers of your opponent. But trashing of Coppers might not be so important in a Province-game, so if you get rid of the Curses you are happy. But you yourself spent some precious $5s to get this Curses in your opponent's deck, and only keep a lesses Silver for it. So it is worth to spend $5 (or even $6s) on bad Silvers to slow the free trashing of the Bishop? Might depend on the $5s (and $4s), but often it is not. To feed the Bishop you also throw away something, the step from $4 to $5 is a large one, but you only get the value of a $4 when Bishopping IGGs.
The board itself is kind of difficult I think, afte the opening Witch supported by IGG I'm not sure. Usually I would say just CR->BM, but with Bishop and Nobles there are significantly more VPs on the board than usual, the Curses already slowed the game, so racing to the Provinces might be a trap. But which way to go? Bishop might be to much of a present if you win the Curses, but that's the only trashing (yes, I've seen Transmute...). But to go for an actionchain without tidying up a litte bit, with Nobles as Village is also not something I feel comfortable with. Opponents Bishop of course might help here... So I think I'm still focussed on CR, but will get disturbed by Haggler and Nobles and lose. Haggler ->Nobles/CR or Gold/CR sounds really good, if it works...