I've used hackberry a few times, I really enjoy working with it. One thing I would recommend is to toast the belly, I think it really improves hackberry.
I think your design would work just fine. I tend to do more of a pyramid style even taper down the limb, but that's just my personal preference. I know a lot of great bowyers here would rough the bow out the way you described. Extra width on hackberry isn't a bad idea, I'd definitely use the whole 2in coming out of the fades.
As far as the handle design, I'm not sure one way is necessarily better than another. I've done both (with shorter limb as top on asymmetrical), and I haven't noticed a performance benefit either way. I usually prefer to center the handle and do 2in above and below though. My eye sees tiller better that way than it does with asymmetrical limbs. It also allows me to flip the bow whichever way I need to if one limb is slightly weaker than the other. But if you prefer an asymmetrical design, that's what you should do.
Can't wait to see what you decide to do! Good luck!