Regarding the whole "Superman should be the Last Kryptonian" issue, I totally agree on that front, it makes the character better and more appealing. I like how they are addressing that by making all other Kryptonians genetically engineered to only fulfill certain roles in their society and only Kal-El biologically and naturally born from Krypton and free to choose his place in life and dream of becoming "something other than what society had intended", thereby making him a true "Last Son" and greater than all others.
I imagine that won't be too different for Kara. Kara would still be a synthetic Kryptonian and Superman would still remain special.
If I had to pick someone to play Kara, I'd definitely go with Amber Heard.