If your reagents aren't precious, I'd just stop worrying and run the reaction, after workup pop it on the NMR and see how much product you made. If its not satisfactory, then you can start trying to diagnose what's wrong.
Can you form your Grignard in situ with Mg metal? That way you don't have to worry about conversion from iPrMgCl. The I2/sulfide method sounds impractical and smelly.