Don't apply 'All' to that discount but instead use the groups where you want it applied. Note that Corporate is by definition a part of All. I don't think it's a bug as you describe it.
The problem comes that I need the qty discount to show for both guests and and people logged in who are not corporate accounts. If can specify group to be guest, but if I specify as registered user, then that also is including all user groups that are logged in.
Yes, registered would also be inclusive of Corporate group. This has been cs-cart functionality since day one. Good luck getting it fixed/changed in the core. Seems like proper behavior would be that when someone logs in and they are a member of any groups, then 'Registered' would NOT apply to them. Maybe a better name would be 'Logged in and no groups".
You'd have to change the logic in the core files to get that behavior. I don't believe there are any hooks related to extending the pricing.
You can use the get_product_data_post hook in the fn_get_product_data function (app/functions/fn.catalog.php) to correct value of the $product_data['prices'] array