For my big grain steeps, I did just what you described. Steep in my BK, transfer to another container and then transfer back to the BK, adding water as needed. Portions makes things more difficult, IMO, and you'll get more even temps with a larger amount of grains.
As for your hops, I've tasted a number of BIPA's (been one of my fav's for a few years now) and I've seen almost everything done to them. From traditional English style hopping, classic West Coast pale hopping to experimental stuff, like bourboning, chocolate and even a port wine barrel age. Heck, it wasn't even a recognized style before two years ago, so there's still a lot of room for experimentation! I'd say as long as you reasonably follow the traditional uses of hops (bittering, aroma, flavor), you'll probably be all right and it might turn out super interesting. As for chocolate, I was mixed on the one I tasted...but it wasn't entirely out of place. I'd say go traditional and maybe experiment later if you love the style.