1/4" is all you need for the sides. I could park my truck on mine and it would support it fine. Using 1/2" on the sides is overkill and just plain silly.
What are you covering/finishing the top with?

I'll be finishing it with veneer stone. I really haven't figured out the top but we were considering either a concrete counter top or granite.

Thanks for your help.

Either one, you are OK with 1/4" on the top.

NO you can not use Hardi for any of it. and NO again 1/4" is rated for floors only not walls.
When in doubt ask the guys who make it and James Hardi says no Hardi backer for exterior applications period none at all. Durock however will work fine.

James Hardi is selling himself short. And I am willing to bet he is not looking at BBQ Islands when he makes that comment. They have a great product.

The secret is in the surface protection and the framing. I have Islands all ver the world made with 1/4" HB. We have been doing it for 15 years with no problems ever.

The plant that does the Barbeques Galore Islands has made over 10,000 that way with no complaints. The recommended surface is stucco and here in California, stucco does a pretty good job of protecting about 50 million wood frame homes.

My personal one is nearly 12 years old and still looks brand new.