If it's length is reasonable yes it's possible will fit.
Really will depend on your mobo and the proper pciex16 slot lining up.
When the dimensions are released measure your case.
The bottom hdd cage is removable in the CM690II.
I'm guessing it is. As it is essentially 2 6970s on 1 PCB. Even if an SLi or xFire is a little faster it scores on using less power, cooler and also future proof since you can add another HD 6990 in the future.