Please comment on your experience with Plantation shutters. Yes they look great,

Well the only practice difference between shutters and glass, or any flat surfaced wall, is the slight curved nature of the blades will give a small but uniform  bit of diffusion. It won't diminish it - how that might sound I don't know. You are right though, it will be better than uncovered glass. If your speakers aren't hot on axis, toe in may still be the best solution

I plan on toe in for sure. My wife will not allow drapes, but she loves these expensive plantation shutters. No magic sound properties it seems! 

In my limited expertise, I think the bandwidth over which shutters diffuse is going to be fairly narrow.

sounds like the decision has already been made

Install drapes that hang outside of/over the shutters.
Keep the drapes open except when you want to listen.
Tell your wife it is a necessary energy saving measure for the cold Minnesota winters.
Good luck with that.